Call your credit card company and tell them when you will be abroad (exact dates) so your card is not frozen. See what your CC and your bank's policy is on exchange rates (Do they charge a fee? Can you use their ATMs abroad?).
A month or so before you leave, order some of the local currency (euros, pounds, etc) from your bank, so you are not stuck with exorbitant airport exchange rates.
Determine how much money you will need/are taking, make a budget, and STICK TO IT. It's okay to treat yourself, but don't go broke!
Don't pack more than you can carry by yourself. Toiletries, towels, etc. can be bought abroad.
Weigh your luggage before going to the airport. Airlines generally charge penalty fees for luggage that is too heavy and may even make you repack your bags.
Consider packing into a smaller piece of luggage inside a bigger piece of luggage. That way you can leave with one bag, but when you are ready to come back to the states, you will have two suitcases to fit all of your new souvenirs!
Bring a gift for your host family. Try for something uniquely American (regional delicacy, sports memorabilia, souvenirs from your home city, etc.).
Either bring a phone that can exchange SIM cards or buy a cheap phone and SIM card there.
Laptops can (and will) be stolen. Consider regularly backing up your photos and other important documents either to an external hard drive or the cloud.
Be prepared to represent the US to other countries. Have some knowledge of current events and politics. It might be useful to form your opinion on them as well.