I've found that drawing with pens, particularly thick lined and cheap pens, is incredibly liberating and rewarding. Something about the fact that there is no "undo" button, that no eraser in the world could take away your mistakes, actually produces more confident lines rather than the logical reverse.
My favorite pens are those that run out quickly and come in packs of 15+ ... good ol' Bic is my scribble-friend for life. :) No, it's not "artist quality," and it probably isn't even really black (most cheap pens have a red or blue undertone) but if all you're doing is practicing and freeing up your hands, you hardly need to be spending over $2 a pen anyhow. (Unless your independently wealthy, and in that case, good on you.)
As you may have noticed, I am a major fan of crosshatching, as well as crosshatching's lazy cousin, scribbling. It's a nice way of breaching the gap between gesture drawing and a firmer, more traditional drawing while staying loose.