Toby brings up some great points as well. Crop a thumb that will stand out from the others. Don't just make it a standard thumb, try to concentrate on an angle, closeup, or something that begs to be clicked. Here are some I submitted I think are good...
http://www.howling-wulf.com/thumbsamples.htm
I want the surfer's eyes to be drawn in and make him think "woah! what's going on here?" *click*
I especially like the thumb with the guy in the background taking a pic with his cellphone lol that cracks me up.
Please note I don't have photoshop I just use Irfanview (free) to crop and adjust colors, sharpen, etc if needed takes < 10 seconds. Great program.
Descriptions too. Like Toby I receive submissions, and am often given snooze fest descriptions like this one
"Two hot galleries of cute girls having hardcore sex with horny guys"
What a waste. It's a freesite of Hardcore Partying galleries, which is basically a huge orgy party. Put in an iota of effort and go with something like
"Celebration party degenerates into a drunken orgy of loose morals and kinky sex."
Took me about 15 seconds for that one. Same freesite, different descriptions, different keywords, different CTRs, different results.