>>1542796First make sure the pads aren't worn out. There normally are ridges in the braking surface. If they're worn through, time for new pads.
If that's ok clean the rim and the pads. Take the wheel off or unscrew the pads to get at them.
Might also help to 'toe in' the pads. Basically mount them so the front end is closer to the rim than the trailing edge by putting a thin coin or something between the back of the pad and the rim while you're screwing it on.
