Here's some tips that I've found that might come in handy for you future AFSers out there:
1. ATTITUDE IS EVERYTHING. Research about your country is great, but there's no way you can avoid being surprised. Being able to take it all in stride and adapt is key to having a good experience.
2. If you're American and used to the crazy non-metric system, convert important numbers (like your weight, height, the highest and lowest temperatures of your home, distances, etc.) and memorize them ahead of time so you can answer questions about your country.
3. For Belgium, pack warm clothes! Especially socks. Long Johns are not amiss either. Humid cold is not to be scoffed at, even by Alaskans. Warm, water-proof shoes are also important.
4. Bring photos of your home that you can carry around with you easily to share. Photos on a computer are great, but it's a lot harder to pass them around a kindergarten class.
5. Learn as much of the language as you possibly can before coming. It will make your life so much easier when you're trying to integrate.
6. Bring clothes that aren't tight. Eating is a way to participate when you can't communicate otherwise, and almost all exchange students gain weight.
7. If possible, get your hair cut before coming to Europe---haircuts here are crazy expensive. Salons charge around 30 euros a pop for women, and men are around 20.
8. Become Yes-man, especially at the beginning: get involved and try everything, even if you are exhausted. It's the best way to integrate and beat homesickness!
9. If you are a picky eater, do your best to get over it. Food is an important way to connect with others, especially if you can't talk much. Cooking for your family is also a great gift and a fun way to share your culture! Americans might consider bringing their own measuring cups.
10. Plan on buying a cheap cell phone or bring one that will work in Europe. Cell phones are just a matter of security nowadays.
11. Do consider bringing a computer; DON'T abuse it and become a hermit in your room. Americans: if you bring a computer bring a plug-in converter.
12. Be up to date on your country's politics and current issues so you won't be ignorant when someone wants to ask you about so and so's stance on such and such.
13. I would definitely recommend American Field Service as an exchange program. I have had absolutely no problems and many successes with them. Simply put, they rock.
14. Take photos, journal, blog, and just savor your adventure! Live it up!