Attached Link: http://www.onona3d.com/tutorials.htm

I'd just like to add in here that ClawShrimp is correct. The best way is to paint it as a separate map, to be honest. These other methods work but not always and not always well. You should take a look at Texturing For Dummies by Leigh Van Der Byl. Handy info there (not to mention free). See attached link.

A great book for texture artists is Digital Texturing and Painting by Owen Demers.  ISBN : 0-7357-0918-1