1: Do make a list of what you need/want.
It's good to have at least a general idea of what you're looking for before diving into a store. It will help keep you from becoming totally overwhelmed by a large amount of stuff!
2: Don't shop on a time schedule.
This is not the time for power-shopping! You should have at least an 2 hours of your time free to devote solely to searching.
3: Do be ready to dig.
Wear comfy clothing, and get ready to crawl on floors and under racks. Most stores have goods from floor to ceiling, and it usually requires several different yoga poses to reach it all.
4: Don't miss a single shelf, box, or rack.
Most stores will have treasures tucked all over, in a variety of different ways. Some of my best finds have been found in the bottom of dirty cardboard boxes.
5: Do try everything on.
With clothing, sizes vary. Vintage clothing has totally different sizing, and newer clothing may be either stretched, shrunk, or just slightly irregular.
6: Don't buy anything that's beyond repair or cleaning.
With a few exceptions (such as collectibles), if it can't be reasonably repaired, painted, or scrubbed clean, then it isn't worth it.
7: Do become friends with the staff.
Although this will naturally happen if you frequent a shop enough, staff members can help point you in the right direction for a specific item, or even alert you as to when new items will be put out, or when the next sale will be.
8: Don't be rude to other bargain hunters.
I don't care if you've just found the Holy Grail of your thrifting career- it is never okay to push another person out of your way, step on someone else's toes, or grab something that another person is reaching for. This is thrifting; not a Black Friday Sale at Wal-Mart. Be nice.
9: Do have a tape measure, pen, paper, and camera with you.
This especially important if you're looking for furniture. Believe me- you don't want to get all the way home with that fabulous end table, only to find out that it's too big for the corner you wanted to put it in.
10: Don't leave something you're undecided about unattended.
I say this from experience. If you can carry something with you that you are undecided about, then do so. If you can't, then sit on it while you decide. I've lost too many goodies that I let sit on the shelf while I thought about them, only to go back and find them gone.
11: Do shop regularly.
Set aside a regular day in your schedule to visit your favorite shops. I try to go at least once a month to all of the places that I love best.
12: Don't forget to keep track of sales and specials.
Most stores make discounts on items after they've been in the shop for a certain amount of time. Others will have bag sales to clear out merchandise. Be sure you know when all of these sales occur to make the most of your trips.
13: Do have fun!
Try on the bad 80's prom dresses. Flip through that stack of random magazines from the 70's. Don't take it all so seriously- have a little fun while you do it.