- Mark constantly
If you mark every time you sing other than during the performance, you actually lose your stamina and your vocal strength. You end up more likely to hurt yourself when you suddenly go back to singing full out.
By marking constantly you miss the chance to create muscle memory, which will make it harder for you to hit the high notes and the fast runs during performance.
- Mark all aspects of the music
Because you give less vocally when marking, you will tend to give less in all other aspects of the song. You create bad habits when you stop paying attention to diction, energy and musicality, so make sure you stay engaged.
Picture from http://blogs.voices.com/voxdaily/conferences/Pict