h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

No matter where I'm going, I've found that the less you take with you, the happier you are. To that end, I've fit everything into one small duffle bag and a satchel. It helps when you're running through the terminal, and all your luggage is carry-on.

In your satchel, of course, you must always have your own towel, to use as a pillow on the plane when the man next to you has all three from your row. Use it to stuff under the door at your hotel next to the highway. Wrap your duty-free retsina so the bottle doesn't break. Have it handy when you're eating and the napkin dispensers are out. And of course, dry yourself out, because hotel towels are made purposely scratchy, so that no one heists them.
Have a plastic insulated coffee mug handy for when your flight's delayed, or any time you get your brew so that you don't waste paper cups.
Buy one small bottle of aspirin, empty 3/4 of it, and add ibuprofen, Immodium, a decongestant and an antihistamine. Swiss army knife, for opening packages and bottles. Pad and pen, if you haven't a PalmPilot. Extra batteries.

As for the duffle, take mostly polyester clothes, because they don't retain odors, dry out quick, and compress better than cotton. And they're far more fashionable now than they were in the 70's. Stuff your toiletries (toothbrush and paste, small shampoo, comb, contact solution if necessary) into a small, preferably plastic-lined bag, and put these in your duffle. Depending on the weather conditions of your destination, wool socks or beach sandles are nice to have along.
1 shirt for every 2 days, 2 pair pants no matter how many days (alternate between them and no one knows), underwear for every other day, and socks to match.

But by far, the most important single item to take with you is patience. Spend a week before you leave building it up, and hold it in reserve for those frequent times that nothing goes your way.