This might be obvious, but after the students get a bit of a handle on how to use the app, (i.e. how to record points, how to read their velocity, energy, etc.,) give your students a little bit of creative freedom.

Let them try tracking different parts of the body of their subject and see how much difference it makes in terms of the quantities they are measuring.

Along those lines, I agree with some of the other tips posted, but it is also important for the students to find some of their own errors.  For example, Mr. Moore mentioned the importance of keeping the iPad/camera still and not panning towards the subject.  That is very important for accurate data.  However, maybe it would be beneficial for the students to try to figure out why their ‘subject who is running at top speed’ is showing a velocity of zero.

Again, I liked to let my students ‘play’ for part of the exercise, after they were done with understanding how to ‘play.’