Making masks with c is the same as in AE. A good way to do is is not frame by frame, but by allowing the computer do the work with interpolation. This means if you set a keyframe for your masks position at frame 1 (don't forget to have the Animate button ON or you'll do al ot of work without dropping a single key,) and another at, say, frame 30, the mask will move between the 2 keyframes. So if the motion of the object is nice and linear, your roto will pretty much be done. However, nothing ever moves in a perfectly linear way, so here's a good way to do it. Keyframes are called keyframes as a reference to clasical hand-drawn animation; these are the frames that mark a point where things change ( put generally,) so what you need to do is set a key for your first mask on frame 1, say your left foot mask. Then step foreward through your footage paying close attention to the foot motion. As soon as it changes direction, drop another keyframe. Do this through your clip. Then you'll probably have to place keyframes in between your original string of keys. Keep making in-betweens until you have a nice smooth roto.
You may have noticed I said to just roto the foot-you'll need to break up your subject into several parts. It's never a good idea to try to make a complex matte with a single spline with hundreds of points. Isolate distinct areas that move seperately of eachother. With a person, it's easy; a hand matte, a forearm matte, achest and shoulders matte, etc. It sounds like a lot of work but trust me it'll save you days in the long run.
Also don''t bother to roto stuff that won't need to overlap over your mid layers. A common rookie mistake is to roto out a whole guy with the intention of putting a cg monster chasing him, but the monster actually only intersects with the foreground subject on his left side. Watch all of you clips and become familiar with them before a big roto gig to save you wasting time rotoing regions that just don't need a matte.
You can also combine keying with roto. Any chance you can pull a luma key from regions you isolate with the Draw Selection tool?
Most importantly, bring a lot of good cds and nice headphones. Roto needs ZEN...
hope this helps,
jon a