The number-one rule in packing for a trip, unless you have hired several football players to carry your gear for you, is to pack lightly. You will be moving your luggage into the trunk of your car, out of the trunk of your car, through the airport, off of the luggage carousel, to the train station, to the bus stop, onto the bus, off the bus, back two kilometers because you overshot your stop, up six flights of stairs, and onto the dresser.
While this might be great exercise, you might rather save your energy for climbing up to the top of the cathedral. Leave the bowling ball at home.
Toiletries
Toothpaste
Toothbrush
Contact lens equipment and spare glasses
You might also wish to consider extended-wear contact lenses, especially if you are going to be going somewhere with poor water supplies (e.g. camping).
Makeup (for business trips)
Razor
Hair care equipment (brushes, comb, blow dryer)
You might wish to consider growing a beard, letting your leg hair grow out, and/or getting a crewcut (yes, women too!) if you are going on a long, low-budget trip.
Condoms
These might be unavailable or difficult to obtain where you are going, especially in Catholic countries.
Business/calling cards
If you are traveling on business, the utility of business cards should be obvious. Small cards with your name and address can be very handy for tourists as well, to give to the people you make friends with along the way.
If you are doing a low-budget trip, you should also bring cleansing products:
Soap (in a plastic bag)
Shampoo (or shave your head and use bar soap)
Towel
If you will be looking at ceilings (like on a castle or church crawl), bring a small mirror so that you won't strain your neck.
Clothes
My rule of thumb is to take enough underwear for one week, or for the length of the trip plus two days, whichever is shorter.
If you are traveling on business, you can probably take two suits and rotate between them. Try to take different colors of shirts/blouses; mixing and matching might fool people into thinking you brought more clothes than you did.
If you are going on an extended trip, pick one color and stick with it. If you are part of Generation X, rejoice, as black travels very well. You can get horse slobber all over your black jeans, and as long as nobody stands too close, you'll look fine.
Plan on inclement weather. It will happen, and everybody there will say, "Oh, it almost never rains/snows/hails/blows/floods like this! This is very unusual weather." In particular, be prepared for it being colder than you expect. A polypropelene shirt is a wonderful thing to take traveling with you: it is light, very warm (even when wet), and dries quickly. Packing a Gore-Tex shell is another good way to save a vacation; Gore-Tex rain pants wouldn't hurt.
(Californians and other desert denizens, take note: what will dry out overnight in California might take two days to dry out in wetter climes!)
I also recommend bringing a pair of flip-flops (also called thongs or shower slippers). Not only can they come between you and the strange things that are growing in the bathroom of the scummy dive you ended up in, but if your shoes get wet, this gives you something that you can wear while they dry.
Miscellaneous
Day pack
If you are doing any sort of sight-seeing, take some sort of small backpack or fanny pack. You will want to carry maps and perhaps phrase books, guidebooks, water bottles, sunglasses, and so on.
Neck wallet or money belt
This is especially important on the hostel/train circuit. Keep most of your money hidden away underneath your clothes. If you are as paranoid as I am, you might even want to go to a two- or three-level system: keep passport, airline tickets, and the bulk of your money in a money belt, about US$50-100 in a neck wallet, and about US$5-10 in your jeans pockets.
Tiny flashlight
This is optional, but you can get really tiny flashlights, and they can come in very handy.
Tape and magic marker
This is very useful if you are shipping a bicycle or anything else in a box.
Earplugs, waterbottle (with water!), chewing gum, food
These can make your plane trip much more enjoyable. Aside from being noisy and prone to pressure changes, airplane cabins are very dry, and you will tend to dehydrate if left in one long enough. The gum helps with depressurization, and airline food is, well, about as good as airline food.
Phone Numbers
Imagine that you got rerouted to Omaha, Nebraska because of the weather, and then got all the runways were iced over. You really want to have the number of Aunt Martha, who lives in Omaha. You won't care that you haven't seen Aunt Martha for seventeen years - anyplace with a bed is better than the airport floor.
Passport
Even if you are inside the country, it is a good idea to take your passport with you. It is light and you never know when your company is going to want to send you to China, the Feds (or Mafia) are going to catch up with you, you're going to fall in love with a sexy Italian, or your rich great-uncle in Ghana is going to die. At a trade show in Dallas, I bumped into some former colleagues who asked if I'd like a quick contract that would involve leaving for Paris the next day. Had I had my passport with me, I would have gone.
Passports can also act as identification if your wallet gets lost or stolen.
Watch with alarm
A cheap digital watch with an alarm is small, light, and very useful.
When traveling outside of the country just call your telco and have your international roaming turned on.
