This article is from the Mountain Biking FAQ, by Vincent Cheng.
-Check if your brake pads are toed in. This means that the front of the
pads hit the rims about 1mm before the back.
-If not, adjust the brake so that the front of the pads is about 1mm
closer to the rim than the back.
-If your brakes are toed in, try scrubbing the rims (refer to "increasing
braking power".)
-If the problem only appears when the rims are wet/cold, a different
brake pad might eliminate the problem.
Peat Bakke [pb@europa.com]
I found that a great way to adjust the spacing / toe in of a break pad is
with a nickle and a penny. Put the penny between the rim and the front of
the pad, and the nickle between the rim and the back of the pad.
 
Continue to: