1. Know your size.
There are so many different sizing charts for men that its hard to keep track let alone know where you fall on the t-shirt/dress shirt/sport coat/suit/shoe/tie/pant/belt chart. But trust me, once you know and your clothes fit properly and comfortably the experience gets a lot easier (that goes for regular shopping too).
2. Be picky, but not too picky
Thrifting for clothes is an exercise in patience. Just because you know what size you are doesn't always mean you're going to find something you like. In fact, chances are something will catch your eye but upon closer inspection it doesn't fit at all. Know what aspects of your choices can be tailored (you can always shorten those pants or have that jacket taken in for a reasonable price). The trick is know how much effort and money you're willing to put into that great find.
3. Take your time
Setting aside enough time is essential to having a good thrifting experience. You really do have to look through just about everything to find that gem you can brag about to your friends. A lot of the time things are in the wrong size or section. Better to have the time to look and be sure you don't miss a thing than not.
To those who know, these may seem like simple and obvious ideas but nevertheless they are key to ensuring a productive and rewarding experience. Happy shopping!!