Just my "two cents" and compiled after my recent flea market excursion on July 4th...
1. About timing. They say that you should arrive early to find the best stuff and that's true if what you're looking for is popular or the flea market is heavily visited by professionals. But, they also say that the best deals can be found at the end of the day because dealers don't want to pack everything up again to haul it home. So, it depends on what's more important to you. I'm an early riser anyway so I try to arrive as early as I can. But not so early that I leave my house when it's still dark outside.
2. Try to find out something about the flea market you'll be visiting ahead of time, especially if you'll be driving any distance to get there. Some flea markets are not what you think they're going to be. My local market, for example, is mostly junk—tables full of new clothing, shoes, beauty products, and CDs. I only occasionally find anything there and wouldn't go at all if it wasn't ten minutes from my house. The great thing about hunting for treasures at flea markets is the accidental great find and you want to travel to places where that might actually happen.
3. Wear comfy clothing and shoes. You'll be doing lots of walking and blisters are not a good thing. I wore a pair of crocs on my last trip and they worked great except for the tiny pebbles that kept working their way into the holes at the sides. Ouch! Next time I'll try sneakers.
4. Bring a big, lightweight tote bag for your purchases. It won't help much for larger items but it's perfect for smaller things—like the buttons, trims, and hankies I like to buy. I have a brightly striped one that is also good for identifying me in a crowd. That way if I wander off my husband can spot me from a distance.
5. Bring a tape measure if you're looking for furniture or something to fill a particular spot in your home. You don't want to buy something that's too big for the space you have to fill. Or too big to fit in your car!
6. Bring lots of small change. Yes, they'll think you're crazy at the bank but vendors will appreciate it if you have exact change. Many won't take checks or credit cards anyway.
7. Don't be afraid to ask for a better price but don't be rude. A simple "Can you do better on the price?" will do just fine. If you buy several items from a vendor they'll usually be amenable to a group discount. I bought a patchwork quilt from a vendor and she gave me the handful of handkerchiefs I had in my hand for free. Chatting up sellers and being nice will go a long way.
8. Stop to refuel. Large markets often have food stands and they can be a great way to sample local fare. Smaller markets don't always have the healthiest choices though. If greasy junk food isn't your thing then consider packing a picnic lunch. We do this for the Bouckville show—not because the food isn't good but because the lines are so long! Make sure you drink lots of liquids, too, especially if it's a hot day. Lemon-Shakeups (my personal fav) are good for this purpose ;)
9. About those liquids, make sure when you arrive that you scout out where the bathrooms are located. You don't want to be hunting for them when you really need it, if you know what I mean.
10. Have fun! Laugh about the silly things you remember (and wish you'd kept) from your childhood. Be open to discoveries but don't be disappointed if you don't find anything. That's the beauty of hunting for treasure—it's like a game—sometimes you win, sometimes you don't.