This is one of my favorite things: have a clothing swap. Our ward (church group) has had a few and I have scored big time. Here's what you do.
*Find a partner in crime. Someone in the same boat you are who is willing to help out. Then find a location. A backyard or church gym works great.
*Invite lots of different people. Especially people who have children around the same ages. This is the key here. If you only invite 3 people, and one of them has teenage children, the other has a newborn, yes, there might be some women's clothes in common, but if you are looking for your kids, you're gonna be out of luck.
*Lay everything out by genre. Set up a table, or a blanket of baby clothes, a table of boys clothes, a table of men's and a table of women's. If you have lots of extra time, sort by size to make things easier.
*Make it known that people should bring things that they don't want anymore, and that all items left will be taken to the local thrift store or charity.
*Look with crafty eyes. I don't mean, try and con people out of the best clothes. If you are a DIY kind of person, look for things that can be made over. Is there a long skirt that would make an adorable dress for your daughter? An over sized t-shirt that would be good for a ruffle skirt or a scarf? How about a boring cardigan that just needs a brooch or a few ruffles? And just think, you are getting it for free, so if you try something and it doesn't work, you're not out a penny.
*Be patient. You never know what you're going to find at the bottom of the pile.