There are a few tricks that can be learned to help good lip sync in a music video, so that it looks realistic and convincing. The first main step is in the filming process. During filing it is best if you get the singer to sing the song over the actual track, this means that their facial expressions, breathing and mouth and tongue vibrations are all the same like when they recorded the song. Also during filming it is always best to film the entire song from start to finish for each scene set up. this makes it easier to edit when you have each shot the complete length of the song. Also make sure the song is playing loud during filming, so when you are editing you can hear the song in the footage.
The second step is to sync in the edit. The best way to do this is by ear and not by eye. By keeping the sound on while syncing the song with the footage you can match up the song until it is identical with the original and the song played during filming, this will guarantee a definite match for lip syncing throughout the entire video. Once you have synced once, you shouldn’t have to do it again, there fore a faster edit.
And remember do the lip-syncing by ear not by eye! If the singer has made mistakes there is not much you can do, but at least you got it on point.