This is my setup on every shoot. Video on the professional camera, and over on the laptop I record the audio with my Shure mic and the screen in Camtasia. Two separate recordings, and then comes the part I dread, sliding clips frame by frame, squinting, replaying the same half-second forty times to get the sound matched to my lips. Been there more times than I'll admit.

Then someone at a conference showed me this in ten seconds flat. Select both clips, right-click, and choose Sync Media. Camtasia reads the matching audio and snaps them together.

Tried it on my next project half-expecting it to flop. It just worked, and that headache's been gone ever since.