Robert Hogan passes along some great editing advice as he works on his film Los Tres Cabrones:
Make the hard sacrifices to keep it short. Just because a scene is pretty doesn’t mean you should spend five minutes on a scene that could achieve the same impact in two minutes.
If your shot contains action that exits the frame, do not linger on it, even for a couple frames. Don’t give the audience a chance to think about when the cut is going to occur.
There’s more at Adventures in Storytelling: On Editing.