I picked my thumbs from the time I was about 19 until I was 27. I stopped about May of last year (2011) and haven't gone back to the habit! It used to be SO bad that it would bleed, and I was always hiding my thumbs in my fists. I was super embarrassed by it. I read a few tips online - tried them - and it worked! Here are some things I did to stop....
1) Whenever I had the urge to pick, I put hand cream on my thumbs instead... I had the cream at my desk at work, so it was there whenever i needed it.
2) I grew my nails out, and got a manicure... It feels really weird to pick skin with a longer nail, so it stopped it temporarily. Obviously this wouldn't work well for guys.... but even if they tried something like cutting their nails super short so it wouldn't be as effective.
3) nail polish: I would wear clear nail polish and pick that off, instead of picking off my skin. It wasn't noticeable to have clear nail polish that was partially picked off... and it gave me something to do with my hands when I had really bad urges! I guess this one doesn't work for guys either... but maybe try something that will occupy your hands like a stress ball?
That's it... It really took a lot of effort at first...and sometimes I would start doing it without realizing! It also looked really gross when it was healing for a while, like a callous I guess.... But in the end it was totally worth it. And I'm really happy it's over with. Good luck everyone!