undo the bolts on the back of the mounting bracket if possible (if your using A64... not sure if theres a backing plate that acts as the "nut" :\) or remove the p4 mounting bracket if your using p4 ..easier then trying to get at the clips, but then again it does require you to remove your mobo.. not like you take your heatsink of everyday though eh
If you have an ath 64 you can unscrew the bolts without removing the mobo. In all honestly the thin is such a bitch to put on I'd hate to have to remove it. Question. Why are you removing it?