The standard for mountain bike brakes is: your brakes should be able to stop your bike within 15 feet while a 150-pound rider is going 15 mph. (And they shouldn't screech or wobble.)
Optimum brake performance also depends on undamaged rims and a true tire alignment (perfectly round rim with no lateral warpage) -- you may need to replace or true your rims to get your brakes working right. This section describes a basic tuneup and cleaning. Please see other sections for brake pad replacement
If your brake cables have dirt in the housing, the brakes may be hard to pull, or the calipers may not rebound away from the tire when you let off the brakes. See the section on cable cleaning.
Release the brake cable by detaching the retaining clip from the cable housing.
If there are local rock-bites on the rim, smooth the ridges off with ultra-fine sandpaper. If the rim is widely rough, clean and smooth the rim with steel wool.
Loosen the stem of the brake pad. On V-brakes, this is done with the nut at the end of the stem.
Toe the brake in slightly at the front end. This means the FRONT end of the brake pad will be a trace closer to the rim than the back end. (Screeching brakes are fixed by toeing the brake pad.) Put a couple of business cards under the back end of the pad, then grip the brake lever to tighten the pad down against the rim. Now tighten the stem fully.
Using the tensioning screw, add rebound to the calipers until they spring away from the rim strongly. Rotate the tire, while watching the position of the pads in relation to the rim. If the rim is not true, both brake pads may hit the rim at different times as the tire rotates. If so, you need to true the rim before proceeding.
If you hear a grinding or feel a rubbing sensation when you apply the brakes (while the bike is standing still), your cables are full of crud. You should perform a complete cable cleanout. If the cable or the housing is bent or damaged, you should replace the cable.
Ride the bike around for 1/4 mile or so, pulling hard on the brakes. Now recheck the pad alignment and lever position.