If you want to use a pencil, something covering the tip, glue, is the best selection.
I dried out a sharpie weeks ago for the same reason. If you use a real one that they see works they'll never question you switching to a dried out one. You can also switch back and have it sitting on the table.
Just remember to not pay attention to the pencil/pen/sharpie, if you draw attention to it they'll suspect it's not ordinary. (same applies to decks of cards, coins, and anything else you might be using in your magic).
-Rik