First I would start by saying I think buying clothing is easier at thrift stores when your kids are younger as they get older the boy selection for sure in slim to none. That said here are a few of my tips and add yours for those that don't shop there often or are thinking about it.
1. Check online for sales our goodwill's have 1/2 off certain colored tags on each day and they do run quarters days on clothes as well.
2. Check in often
3. My biggest tip would be find one located in a area where the houses cost more I find the best name brand stuff at ones located close to these.
Here is what I got on two recent trips. Obviously it's easier to find little girl stuff all are name brands addias, gap, childrens place, old navy and one ur It wich a local department store sells. I got all these pieces for $1.19 a piece on the half price colored tag sales.
9 pairs shorts
3 t-shirst one tank top
1 gap skirt
8 pairs of capris
So 24 pieces for under $30.00!!! Could not have gotten those name brand even on sale for that price.