1. Be prepared to fight
Yup, there will be fight. It will either be over beer or a first cousin. It never fails. All you have to do is remember how to fight. Punch, kick, stuff like that. Always remember to block. Food fights also count as fights so be ready for one of those too. Always be on your guard, stay on your toes, especially against your spoiled little cousins who like to kick everybody in the shine, because there parents forget to tie there leashes to a damn tree.
2. Get ready for hugs
Everyone will hug you..a lot. Especially older people who haven't been outside of the retirement home in a year...you know since the last reunion. Just expect things to get uncomfortable for you, nothing like a bear hug from a grandpa with a Viagra addiction. Awkward. Oh an all of your cousins, who just by chance do not want to be hugging you either, will be told by there parents to give you a hug.
3. Be prepared to eat
You need to brush up on your eating skills. Oh yea skills. My family makes the best freaking food in the damn world. I get mad when I get full. I would just not eat for like month before you go there. That's one route you could take. Then there is the other way. You could start training your stomach by eating way to much until you pass out, then move forward from there.
See not much to it, you should be fine, and if its not just close your eyes and cry for a while. It will be over in no time.