In Dorothy Perkins in Chichester, they currently have buy one get one free on their sale items....don't know if this is store-specific. Possibly not?
Drawbacks - a lot of their stuff is overpriced anyway so brings it down to a reasonable price! Also, you have to take both items back even if you want one item and not the other.
On the plus side, I managed to pick up 4 pairs of jeans for £14 which didn't seem bad.
Student discount available too if you have it.