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

If you're going to Jamaica: (or any third-world country enamored of things American). Bring a cowboy hat or two, Levis, and cartons of Marlboros. There's a limit to how much of this you can bring into the country, of course. You also have to be good at bartering.

For anybody with a severe allergic reaction to anything: the Epi-Pin. This is a little device designed to inject you with antihistamine - enough to keep that reaction from killing you while they get you to the hospital.

I also recommend the basic supplies that you'd have around the house anyway - antacids, aspirin, Imodium/Pepto-Bismol, Band-Aids, rubbing alcohol and antibiotic cream, etc. It may seem like overpacking, but think of it as insurance. In my experience, if you bring it, you won't need it.

Things to avoid:

When in Mexico, drink the cerveza, not the water. A street vendor will sell you bottled water, but it's likely that he filled it up at the tap in back, instead of purchasing it sealed from a bottled water company. If you must have water, look for one of the major brands, and purchase it from a grocery store.

When in Jamaica, DO NOT buy marijuana from anybody at the port or airport. Here's what happens: A Jamaican will offer to sell you some pot. You purchase the pot. Then the Jamaican alerts the officer of the law (who just happens to be standing nearby.) The Jamaican gets a reward for turning you in, plus the money you paid him for the pot. Then he gets the bag of pot back! Buying pot is as illegal in Jamaica as anywhere else, so if you decide to break the law, use discretion.

If you're bringing a number of different prescribed medications, do not mix them up in one bottle. All medications must be clearly marked and in their prescription bottle. They probably won't throw you in jail, but it'll be a hassle if it's discovered.