Mathias Möhl published two Video Tutorials on how you could use his After Effects script Tracker2Mask. Tracker2mask can help you when you do rotoscoping in After Effects.

“Tracker2Mask computes the movement of maskshapes based on the movement of trackpoints. It is easy and flexible to use. In particular you don’t need one trackpoint for each maskpoint. In the tutorial I show how to rotoscope the shape of a moving car based on only four trackpoints, for simpler shapes often one trackpoint is sufficient.”

Tracker2Mask Tutorial from Mathias Möhl on Vimeo.

In the second Tutorial Mathias Möhl shows you some of new features  like allowing local modifications of masks.

Tracker2Mask Tutorial 2 – new features from Mathias Möhl on Vimeo.

You can DOWNLOAD tracker2mask for free at (right click->save target as)
ps.uni-sb.de/~mmohl/aescripts/Tracker2Mask.jsx