Even thrift stores have sales!
The key to taking advantage of these sales here in Marion (and probably other small towns as well) is to make regular visits. I try to make a thrift store trip once a week during my regular grocery shopping excursion. Will I miss some sales this way? Certainly, but I'm okay with that because I'm shopping when I need something, not just for the sake of shopping. If you're an eBayer or just enjoy looking for deals, you'll want to stop by a couple more times in the week.
The sales for my local stores are posted inconspicuously on the interior entrance doors of the building, for example, "All purses 25% off." There are almost always sales right before the holidays as well, just like any other retail outfit. I've called all the thrift stores I know of here in Marion to ask if they had periodicals or newsletters or any other form of notifying customers of their promotionals, but I got a "no" from all of them. The lady I talked to at Goodwill, in fact, insisted that they do not run sales because of their "everyday low pricing," but I happen to know that isn't true. Instead, I'll keep stopping by and pay attention to the postings.
If you're in a large city, you may be able to find sales postings for your local thrift stores online because these stores have more sales competition, but even many of these large retailers have no outside promotional postings.
Keep your eyes open!