EDITED TO ADD -- as far as I know, this only works with a single chainring / no front derailleur.

Veloorange chain guard, ~ $30
http://store.velo-orange.com/index.p...hainguard.html

The bashguard-like things that mount to the chainring don't prevent your pant leg from getting slimed by the chain in the vicinity of the 10-11 o'clock part of the chainring as you're pedalling through the bottom of the stroke. A pant strap is still needed with one of those, but not really with this: