Perhaps one of the most fundamental skills a filmmaker needs to have down pat is the ability to focus well. This gets all the trickier when you or the subject are moving. This helpful video will show you four ways to keep your subject in focus and walk you through various scenarios to show you which method works best in each situation.
Coming to you from Parker Walbeck, this video will show you how to maintain subject focus when movement is incorporated through four different methods: maintaining a constant distance, handheld continuous focus pulling, using a narrow aperture, and autofocus. Of course, each has respective drawbacks and benefits; whereas using a narrow aperture results in a deep depth of field and thus, a wide margin of error, it eliminates the subject-isolating effect of a wide aperture and isn't really an option in low-light situations. On the other hand, some autofocus systems perform better than others, and that performance can be highly dependent on the situation (Walbeck is a fan of Canon's widely lauded Dual Pixel Autofocus for this method). It all depends on your skill, the look you're going for, the equipment you have, and the shooting environment.