Stretch! No, really - stretch!
As I was in the midst of the first half of my 60 minute before noon practice routine (I broke it up into two 30-minute sessions today, because I felt like it), I realized that I didn't stretch. And I paid the price.
So, guitarists, bassists, and all musicians alike, do yourselves a favor and get into the habit of stretching before you practice. I do a couple minutes of wrist flexing and extending on both hands. Then I do circles, five times with each hand in both directions. It takes about three minutes at the most, and the return on investment is wondrous.