Hmm, seems like these can be pretty different depending on who holds them. The few I have attended have been indoors, in the back storage rooms. They have been crowded, so I'd definitely agree with leaving small children at home. Actually, go it alone or with an adult friend so you can totally focus on checking out the bargains. Different from Ollie's experience, I have managed to find some slightly worn boots (looked like they had been worn for a weekend trek at the most) that are still my primary hiking boot three years later, and a brand new pair of cycling cleats (they were the prior year's model that they were clearing out). So, while I had a different boot experience, I agree with the rest of Ollie's recommendations. I might add one more:

Call ahead to see if it will be held inside or out, that will help make sure you are dressed appropriately.