You can use Alan's way with the effect Effect>Distort>Corner Pin, and keyframe each corner over time, or you could use the perspective motion tracker, which tracks the footage and basically keyframes the corners for you. However, depending on the footage it may be easier just to do it manually with Corner Pin. Here's a tutorial on the motion tracker.http://studio.adobe.com/us/tips/tip....aft65cornerpin
Hope it helps. Peace