I also use it as an incentive to get the kids to purge their toys. I help them price everything, and anything that sells, I give them the money for. Last fall they each earned $9 and they were psyched, even kicked themselves for not selling more. I look forward to picking up some "new" clothes for the kids at the actual sale. I still have yet to sell more than I buy for, but even so the savings are amazing.
With my first daughter, I bought almost all of her clothes new and shunned handmedowns and sales like this, now I love them. Kids grow so fast, consignment sales are the way to go when getting stuff for your kids. I posted some tips after the previous sale in the fall - check it out if you're new to consignment sales