~This is FAR from being a complete list but here's few mistakes I've observed made by newer comedians. ~ Take heed and you can scratch these off!
Get it out of your way. If you take the mic out of the stand don't leave the stand directly in front of you. Move it behind you or off to the side.
Careful as to how loud you are on stage. If the room is noisy and/or the mic isn't very hot (loud) then you might have to put your mouth closer to the mic and really project your voice. If they can't hear your material, it's not funny.
If the mic IS hot be aware that you may not have to get it as close to your mouth or project as much. Pay attention to the people ahead of you to gauge necessary volume. Being too loud is as bad as being to soft spoken on stage. Both can be annoying to the audience and reason to tune you out.
The mic should for the most part be held comfortably under your bottom lip. Refrain from holding the mic as if you're making announcements at a flea market or performing as a rapper.
*WATCH TOO MUCH and UNNECESSARY STAGE MOVEMENT
When arriving to the stage strike your comedic pose and start into your opening joke. Careful not to move constantly, don't walk around too much, no rocking back and forth or fidgeting or turn too much to one side and then the other. Never turn your back to the audience. All of this signals nervousness and ill confidence. If you act confident your audience will follow your lead and relax.
Standing still most often will begin to settle the audience and draw them to you. Standup comedy is much like acting...less is more!