Trey Morgan started an extended tutorial on motion tracking with Mocha for After Effects, Advanced Motion Tracking Part 1. In part 1 (59 minutes) he starts with an relatively easy screen replacement to show basics, then he'll move on to a more challenging clip to show how to deal with obstructions, tracking planes moving off screen, etc.
You can more info and tutorials on Mocha AE in AE Help, and a different and more immediate mix of resources for Mocha AE and tracking in general at the old AE Portal. Here's Trey:
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