I'm missing JF for the first time in almost 20 years...
I hate that I can't make it this year... but here are a few of my top tips:
Whatever day/days you go, pick the big act that you want to see at the end of the day and set up a spot at that stage early in the day. For example, with Pearl Jam, bring a couple of camp chairs and set them up at the Acura Stage as soon as you get to the fairgrounds. You can wander off for a bit earlier in the day and people won't mess with them, especially if you make friends with people around you. But I'd also recommend getting back to that stage an hour or two (maybe more) before the headliner starts. If you try to get to Acura later in the day, or worse, right before Pearl Jam's (or any other headliner's) set starts, you'll end up packed into a crowd and not able to move.
It's tempting to just booze it up all day but drink plenty of water, too. You can thank me later. I've seen people suffer from the effects of dehydration and the EMTs can't get to them (see above about the crowds).
That being said, sneak in some vodka and buy a strawberry or raspberry lemonade. Mix and enjoy. Also, bring a small soft-sided cooler and buy plenty of drinks at once, especially later in the day. If you buy enough beer and tip the folks at the beer booth, they'll usually give you some ice for the cooler.
I can't stress enough how bad the crowds are at the Acura Stage and the less you try to move around later in the afternoon, the better off you'll be.
Almost all of the food is good but try the Crawfish Monica. It's a Jazz Fest staple. The red beans and rice are surprisingly good, too. I'm drawing a blank on the specific name but there's also a chicken and tasso curry-like dish that is fantastic (it's usually near the Crawfish Monica booth).