Don't be a wallflower, Look for a table with a few empty spots and some Germans that appear to be fun/partying. Buy a round of beer for them. The best days/nights we had were partying with locals.
Take a night off from the fairgrounds and party at Hofbrauhaus (downtown)
Don't start or be involved in a fight or you'll be gone instantly.
Drink lots. I don't remember going home one night and I woke up without a hangover every morning, it was amazing.
Get into a tent early and stick with it, don't be tempted to move around or you might not get in again (the party tents fill up early)