Take the cap off the pen with the right hand and turn to the right so that the audience is looking at your left side.
Place the cap in the left hand and slide the pen behind your right ear, then come back to the starting position before placing the cap behind your left ear, and pretend to be disappointed after realizing the pen is gone.
Turn and show everybody that the pen is behind your left ear as you grasp the pen cap in your right hand. Laugh heartily with everyone else.
Pretend to shove the pen into your left ear. The idea is to give the illusion of the pen entering the ear. Put the pen into your ear sideways or take the more difficult approach and slide the cap behind the left ear as you pull out the pen.
Tap three times and pretend the cap has disappeared. Now, half the people are going to be amazed that the cap disappeared. The other half are going to realize immediately that it's behind your left ear.
Turn to show that it's behind your left ear, take the pen and slide it under your watch or into your sleeve. You have it here. As everyone's laughing, point out that it's there.
Take it out and there's the cap. Say, "I've never mastered that cap. But unfortunately you all weren't paying attention, and now the pen's gone again."