Cantilever Brakes - Cyclocross
Well configured cantilever brakes should give you good stopping power. Start by making sure that the braking surface and the surface of the pads are clean. You can clean the braking surface with steel wool, and the pads should just need to be roughed up with a coarse sand paper.

Make sure that your pads aren't below the minimum wear line.

Cantilever pads are harder to adjust than most other kinds of bicycle brake pad, but the principle is the same. The pads should rest one or two millimeters from the rim when not engaged. See here for a better run-down on how to adjust them than I could supply:
http://sheldonbrown.com/canti-trad.html