I use these.

http://www.zebrapen.com/ball-tele.html

The Zebra telescopic pen comes in a two-pack with two refills...quite handy. Closed, it won't come open in your pocket; opened, it writes just like a full-size pen.

Just don't lend it out or you may never see it again. My employer wants to put our company logo on one...they're about $4 for two pens.

Cheap, and worth it.