Packing

First principles, Clarice. Simplicity. Read Marcus Aurelius. Of each particular thing ask: what is it in itself? - Hannibal Lector

When it comes to packing, remember to KISS-keep it simple, stupid.  Less is more.  Most people overpack.  While being prepared for every eventuality isn't always bad, the issue isn't what could go wrong, because anything can, but what is likely to go wrong.  

Packing really depends on where you're going.  You will pack different things for a trip to a Mexican beach resort than you will to go to Europe for a month.  The key is to identify your planned activities and pack accordingly.  The best strategy is to prepare for simple things, and anything that's a bigger emergency, take enough money to buy your way out of a jam.  

This is especially important with how much airlines are charging for bags now.  Overweight bags are super expensive.  If you're close to the weight limit before you leave, remember, you're going to be buying stuff and have to bring it back.  

A few things to always take

• Lightweight rain gear, even if it's just a poncho or an umbrella.  It really sucks to get soaked, but you want to be aware of how heavy and bulky some of this stuff is. 
• A simple first aid kit, nothing fancy, just a few bandaids and moleskin, some basic medication (asprin, anti-diarrhea pills, anti-motion sickness pills, and of course any perscription medications you need).  You only need to cover the basics, things that you wouldn't go to a doctor at home for, like a small cut, or a foot blister (this is important if you're going to be walking).  Anything else, you can get where you're going without a lot of effort.  Most hotels will even give you something if you need or forget it.  It seems anymore, there's a Walgreens on about any corner in the US anyway.  
• Clothing, this is where most people go crazy.  My basic strategy is to load up on extra socks and underwear, but buy t shirts along the way as souvenirs and wear them while I'm touring.  If its something you're taking "just in case" then t's something that you can and should buy if that case arrives.  If you're going to a popular destination, somewhere not remote, then you can find about anything you need fairly easily.  Walmart is global, and it's not the only store like it.  
• Toiletries.  What do you really need?  I take the basics, toothbrush, shaving stuff, comb, deodorant, and foot powder (from my army days, gold bond powder is a life-saver when you're walking a lot).  I don't use a lot of hair products and like to keep my hair short anyway, but I would highly encourage you to adopt as low maintenance of a hairstyle for your trip as possible.  Unless you're actually trying to impress someone, you're best off being just clean and well-kempt, that spending lots of time and effort on your hair.  Hair products are bulky and heavy and things like curling irons need voltage converters outside North America, which is just one more thing to have to take with you.  Most hotels the world over have hair dryers. On short trips, I often will take a toothbrush, but anything liquid that may draw the attention of the TSA, I buy when I get to my destination. I also do not take shampoo, or soap because about every hotel in the world provides it.  Some hostels, though, and you are on your own.  If that's the case, I'd just buy some when I get there and toss it before I leave. For women: I'm not female, but I do understand the need for feminine hygiene products.  Only advice I can offer, it really depends on how picky you are.  These things are available all over the world, but it may or may not be a brand or style you like.  So if you're picky about it, take your own, if you're not, buy them there. 
• A money belt or similar item.  Nothing will ruin your vacation faster than having your wallet or passport stolen.  Trust me on this, I spent my last day in Prague after living there two years at the embassy replacing a stolen passport.  It sucks. On the same token, if your hotel has an in-room safe, USE IT!  Only take the cash you intend to use for the day.  You won't blow your entire trip budget on the first day, and if it gets stolen, you don't lose your entire trip budget.  If you don't have a safe, at least have luggage locks.  You can get simple ones for just a few dollars online.  This won't prevent a determined thief, but it will keep a hotel maid from taking something that she doesn't think you'll notice is missing. They also keep your luggage from coming open during transit. Keep your valuables in the locked case when you're out of your room.  
• Sun protection.  I am one of those who burn before they tan.  I always take my sunglasses, because I love them and I either take a good hat for sun protection, or I buy one there as a souvenir.  I particularly like taking a cowboy hat to Europe.  Europeans have a fascination with cowboys and the American West and every time I've taken one, I've had someone want to trade me for the hat, usually a soccer jersey because I collect them.  
• Quality walking shoes.  I always walk way more than I think I would on vacation.  Take care of your feet!  I like nice running shoes (My preferred brand is Hoka), but I also like to take my Ecco sandals.  Whatever you take, make sure they're broken in.  Getting a bad blister on your first day of vacation sucks.  Also, socks.  Make sure you bring plenty, so if your feet get wet, or sweaty, or anything, you can easily change them.  
• Backpack or fanny pack.  I have a few options here and it depends on the trip.  Keep in mind, most museums, you'll have to either put it in a locker or check your bag.  I've never had a problem with this, but it does freak some people out.  For hiking, I have a pack that has my hydration system, my hiking first aid kid, and some essential emergency supplies.  It's a lot bigger pack than I want to carry around a city.  I have a fanny pack also, it carries most of what I want, but isn't large.  I usually take just a day pack, like one the kids take to school.  This also serves as my carry-on bag.  I always put a change of underwear in it and a change of socks.  Yes, if you can't tell, I'm a big proponent of taking care of your feet.  Also, it's a good idea to be prepared in case the airline loses your bag.  I can deal with with missing other clothes for a day or two, but underwear and socks are a must.  I put essentials in this bag, because, while the airlines are usually good at getting checked bags to you, if you bag does go missing, at least you can deal with things for a day or two until they either get your bag to you or they give you money to replace what's gone.  It's also convenient to put whatever you pick up along the way in it, whether t shirts, or other souvenirs.
• Duct tape!!!! Duct tape is like the Force, it has a light side and a dark side and it holds the universe together.  Don't leave home without it.  You don't need a giant roll, but some is always good.  I don't know how many times I've had to use it, but almost every trip I end up needed some.  I've used it on a cruise when we got into rough seas to keep the cabinets in our stateroom closed.  I've fixed torn bags with it.  I've used it in place of moleskin on my feet where I am starting to get a blister.  I've used it to waterproof a bandage (but remember it's not sterile, so don't use it directly on a wound.  There is always a use for it and if you need it, you usually need it immediately. Look into some of the fold flat duct tape kits also, instead of a bulky roll.  For traveling, they're much more convenient.  

Things to sometimes take

• Swimwear.  How often do you really plan to swim?  This depends on your trip.  If I'm going on a scuba dive trip, this will be about all I'll take.  If I'm going to Europe, I won't take one, unless I'm specifically planning on going to a beach.  I don't like motel pools, so I don't plan on swimming when I go on a road trip.  So this one depends.  If you're staying in a hotel with a pool and think you may swim, then take it.  I usually don't bother unless I'm with my kids.  
• Clothing beyond casual wear.  This will depends as well.  What sort of vacation are you going on?  Will you go to a place that has a dress code?  Make sure you do this much pre-planning.  If you go to Rome and see the Vatican, they require clothing that is not low-cut, sleeveless, shorts or miniskirts.  Some places in Israel, require women to wear a dress or skirt.  Some higher-end restaurants require men to wear a jacket and women to wear a dress.  Formal nights on cruise ships, and depending on the cruise line, even regular dinners in the dining room require at least business casual clothing.  Plan ahead.  If you don't need it, don't take it.  

Other things to keep in mind

• Electronics.  This depends on what I'm planning on doing.  I take my phone regardless, because its 2017 and everything is on our phones now.  Remember though, international roaming, especially international data, is expensive!  So what do you need your phone for?  With your average smartphone now, if you're on a wifi network, it does not go against your cellular data.  This is useful because when you are on wifi, you can use it all you want without racking up a big bill when you get home.  It's free to FaceTime someone if you're on wifi.  There are free wifi hotspots all over the world, and a lot of tourist destinations have wifi coverage as well.  The Reijksmuseum in Amsterdam has an award winning app that explains the exhibits, for example.  For charging, all you have to do is buy a plug adapter that fits the country you're visiting.  USB connections have regulated voltage, so the wall plug itself adapts to what your regular USB cord uses.  You can buy one of those wall plugs either before you leave or after you get there for only a few dollars.  For computers, most of the cords convert whatever incoming voltage to the output the computer needs, so all you have to do is buy the right plug adapter for your model.  For my Macbook, the plug just slides out of the cord adapter and you slide a new on in.  I think it cost me about $15 for the adapter.  Also, if you're taking a laptop, you can always plug your phone into the USB port to charge.  
• Other electrical appliances.  I generally leave them home.  You will need a voltage converter and those tend to be bulk and more on the expensive side.  They also tend to burn out, especially if you don't pay attention to the amperage.  In a lot of cases, if you're using something like a curling iron, it's cheaper to buy a cheap one when you get there that's already set for the local voltage and plugs than it is to buy a converter, especially if you burn it out because you're pulling too many amps.  
• Liquids. You should know the TSA rules.  Yes they suck.  If you can check a bag, you can put things in a checked bag with a lot less hassle, so this is always something to keep in mind. I will put a plug in for solid shampoo.  It's great stuff and not liquid so the TSA can't complain.  
• First aid.  I like to take a few basic items, band aids, mole skin, pain relievers, anti-diarrhea stuff.  Unless you're going somewhere remote where emergency services are not at ready call, then 

Trip dependent things

• Heavy coats or sweaters.  If you're going somewhere cold, where you need it, take it.  If not, don't.  
• Sports gear.  I am a scuba diver, so, while I can rent stuff at my destination, my personal gear is always way better, so I take my own.  Since I'm going to spend most of my time in the water, I can take way less other things like clothes, so the weight evens out.  When it comes to hiking, I already mentioned I have my backpack I use for that and I make sure to take it.  
• Snacks.  On a road trip, it's an always take, since you don't have to worry about airport security or weight.  On an trip where you fly, it's a never, or at least not until you get to your destination.  
• First aid.  While there's a small kit I take on every trip, if I'm going somewhere remote, like hiking, I have a different kit with more stuff in it.  Think of where you're going, because some of these things are heavy and bulky.

Here's a good way to figure out if you have too much.  Pack everything into your bag and take it downtown with you.  Drag it around with you for a few hours while you go about your business.  There are more times than I'd like to consider where I've done just this while on vacation.  If you don't want to haul it everywhere you go, don't take it.  

I'm sure there's some things I'm missing, and some more specific stuff for different types of vacations.