When I was in a local T-Mobile store yesterday looking for Dell Streak 7 accessories, the guy ringing me up told me that I could save 30% on my entire purchase if I bought another item.
I was already buying the new home dock (compatible with both Streaks) and the gel skin that only seems to come in purple, so when he asked if I had the screen protector, I figured it was because that was the only other Streak 7 accessory that was on display. I told him I didn't want one (I hate screen protectors on glass screens) . . . and then immediately went to grab one after he said, almost under his breath, that I could get 30% off everything if I did. He also mentioned the store's 30-day return policy, which is something I'm sure to make use of soon.
This "buy 3, get 30% off" promo wasn't advertised anywhere in the store, but it's clearly printed on my receipt. I also just found evidence of the in-store special on T-Mobile's website and saw that "buy 2, get 20% off" is supposed to be part of it. The deal is only available at "participating locations" so there's no guarantee that your local store will give you the discount, but it's definitely worthwhile to find out, especially because it's not limited to Streak 7 accessories.