When flying, it is a question of travelling light, while still carrying the essentials.
On your person, at all times
Passport
Ticket
Coins (for payphones, tips)
Cash (not too much)
Credit card
Napkins/tissues (think "No toiletpaper?? I find out now?")
Carry in a purse around your neck, inside your clothes.
Carry-on bag
Clothes for next day (think "lost luggage")
Toiletries
Carry-on bag, small enough to hang by your airline seat
Book
Address book
Pen & paper
Chocolate/candy (energy)
MP3-player (+ enough batteries!)
Painkillers
Peptobismol (tablets)
Small plastic bag for garbage (and if you get sick!)
Earplugs
Sleeping mask
Comb
Camera
Cellphone (but turn it off when boarding!)
Map of airports you are visiting. Sometimes available online.
When flying
Wear very comfortable clothes.
Go to the bathroom before you board. You could be stuck for hours inside the plane, before it takes off.
Put the small carry-on inside the big one, before you board. Take it out before you stow the big one away.
Don't wait until the last minute to board. You'll find it harder to stow away your big carry-on. Stow it, so you can easily get it out.
If you stow your coat, put the coat in a plastic bag. It will protect the coat, and make it easier to get hold of.
Check where the emergency exits are. All of them.
When you have found your seat, take off your shoes and tie them to your seat.
Get an extra blanket.
If you are blase about looking out the window, get an aisle seat: Better leg room, easier bathroom access.
Get up and stretch once an hour.
Go to the bathroom when you can, not when you desperately have to. That's when the line will be the longest. The line will also be long, shortly before the plane lands.
Don't wear perfume, but shower as close to departure as possible. Use a neutral deodorant.
When everyone else jumps out of their seats after landing, take your time and put your shoes on again. Double-check to see if you haven't forgotten anything. Don't keep things lying around; when you don't need something, put it back in the carry-on again.
Leave trash as neatly as possible.
Suitcase
Mark your suitcase with something you can see from far away. Mark it on as many sides as possible. Stickers, colourful luggage belts, but nothing that can be torn off.
Address labels should be taped to the suitcase. Leave your address on a piece of paper in the suitcase, too.
Don't travel with fancy suitcases. Get something that can take a beating.
Don't lock the suitcase.
If you bring something that can break, put it in the center of the suitcase, and use socks and sweaters to protect it. Do this especially if your suitcase isn't hardcover.
Take photos of the contents of the suitcase, for insurance purposes.
Pack at least 24 hours before you leave. Don't wait until right before you leave to pack the "last".
Hotel
Don't bring whole bottles of shampoo/body lotion/etc. Fill up tiny bottles. Take the shampoo containers you get at hotels with you for later use. Get them for your friends, too.
There is usually a bucket for ice, and free ice. You can make your own "fridge" overnight, by filling up the sink with ice (put the plug in first).
Locate the nearest drugstore. Get some sodas, crackers etc., if you get the munchies.
Lock your suitcase when you leave the room. Use the safe, if there is one.