Tip 1: Have a high tolerance for fakes, because if the price is too good to be true it's probably fake.
Tip 2: If you're buying on the internet, assume it's fake. There are no insanely-good deals on authentic bags on the internet. The best you can hope for is a modest discount from the few trusted sites for authentic designer items, such as bluefly.com, eluxury.com, yoox.com, etc.
Tip 3: If you're buying in a thrift shop, assume it's fake. Thrift stores aren't like they used to be. They pay close attention to what's coming in and make sure to identify any highly desirable items. The more urban the locale of the thrift shop, the more likely a bag will be either authentic and expensive or fake and cheap. If it's cheap, assume it is fake.
Tip 4: Even if you do find a real one at a great price, get used to people asking you if it's fake. And never, ever say how much you paid for it, or be prepared to be told "Oh, it must be a fake at that price."
The moral of the story is this: designer bags are heavily marketed and highly desired nowadays. With all that competition, the chances of you finding some treasure trove no one else has found are very, very slim.