→ Never set out to find something specific. If you go to the thrift store seeking an ugly Christmas sweater, or a full-skirted, puffed-sleeved vintage bridesmaid dress for your Glinda the Good Witch Halloween costume, you will most likely wind up frustrated at best and crushingly disappointed (and costumeless) at worst. Instead, buy the glorious Christmas sweater when you happen upon it in June. Buy the ridiculous, perfectly fitting bridesmaid dress because it's awesome and it fits, and find a use for it later (or just sell it on eBay).
→ Shop for sweaters in the summer and tank tops in the winter. Though it's harder to look at sweaters when it's sweltering, the selection is going to be less picked over than it will be in, say, September when everyone is jonesing for fall clothes.
→ The unidentifiable stain on the otherwise awesome dress may or may not come out. Accept that. Thrifting is about deals, and is also about occasionally cutting your losses. And also, about sometimes wasting $4. Sad but true.
→ Keep a running list of items you want. If you remind yourself of that “wish list” when you enter the store, you're less likely to scan past something that you forgot you were looking for in the excitement about the other random items you scored.
→ No matter how cheap, don't buy things that need to be altered unless a) you can and will alter them yourself or b) you trust yourself to actually take things to a tailor, instead of just making a giant pile of things that you intend to take to the tailor but instead end up redonating to the thrift store three years later.
→ Go with your gut. Sometimes, you just really, really love that weird, giant Hawaiian caftan. And then, years later, you're pregnant and find yourself actually wearing caftans. Aren't you glad you stocked up?