the bigger the thrift store, the better. look in the mens section, and don't be afraid to look in all the sizes. i tend to only hit specific sections of the thrift i know will generally be good-- in my case i tend to only look at mens longsleeves/tshirts/jeans and skip the rest. go frequently-- not every thrift trip will be good. i'd suggest bleach painting and customizing your clothes as well.
lastly, try and look for fashion pieces that aren't currently popular (out of season, not trendy, etc) or have a smaller following that are frequently available-- last year, affliction style tees and longsleeves were nowhere as popular as they are now and i managed to snag a few mma elite tops. i recently found some great flannels and henleys because it's summer and no one cares about them right now :) hoped this helped a bit