woodslinger
TreeHouser
I get them from rock and tree climbing, and mtn biking as well. What I have found most effective is pulling back on my fingertips to stretch the muscles and tendons. I start with all my fingers together and hold it for at least 10 or 20 seconds (you'll feel it pull in your forearms) then I do each finger individually. When I used to race downhill mtn bikes I did this all the time, while driving, watching tv, etc.