Don't go over dressed. Wearing white does the trick. Walk around. Open with this question, "what ya'll cookin'? Smells/looks good." If you are drinkin', you won't pay for one all day. If you are eating? Oh lord.
Doing both? Yeah. Been there, done that.
Pretty cool people if you approach them right. Find a cooker and compliment the guy running it...ask him what to do? You admire a guys cooking apparatus at a football game? You can point in any direction and say, "the guy over there said this about your..."
"Who? Oh I don't know. I'm a little lost...it was over there. What is that?
Literally, Darren. I've been there a few times. That approach doesn't fail.