I'm putting together the next Breen's Bungalow -- a QuickTime tutorial that appears each month on the CD bundled with newsstand copies of Macworld (and available to subscribers as part of an online "virtual CD") -- and I just invoked one of my favorite cheats. It goes like this:
Suppose you've got a video clip that runs a bit shorter than the narration that accompanies it. You can't allow the clip to fade to black while you're nattering on about what the clip displays, so you must find a way to extend the clip. My solution for doing so is to add a still clip.
To do so, move the playhead to the last frame of the video and choose the Create Still Clip command from the Edit menu. This places a picture of that frame in the Clip Cubby (or whatever Apple calls it).
Drag this picture into the timeline directly after your clip and size it so that it ends where you'd like the clip to end. Add the transition that you'd normally place at the end of the video clip to the end of the picture.
If you have the right kind of clip -- a QuickTime screen capture or a scene with little motion -- and that picture doesn't stay on screen for too long, your audience may never know that you've added a motionless extension to your video clip.