Most mountain bicycles sold today have a suspension, which generally consists of a suspension fork with anywhere from 3 to 10 inches of travel, and more expensive bicycles have a rear suspension. The primary purpose of a suspension is to absorb the shock created by various types of terrain and keep you in control of your mountain bike.
Figure out what type of suspension you have. Inexpensive suspensions sometimes only consist of springs built into the blades of the fork without any compression damping. More commonly, forks will use springs or air for compression with oil or air used for compression damping. This damping mechanism is generally adjustable, and adding more oil, or thicker oil, will increase the damping effect. Similarly, adding more air to an air fork will result in a firmer suspension.
Familiarize yourself with the external controls of your suspension. Often, if something about your fork feels "wrong," there's nothing actually broken or in need of repair; the external adjustments on your fork can change the geometry and feel of the ride. On some forks, a tension adjuster is located at the top of each fork blade, dictating the height at which the bike rides and the amount of travel. If your fork has external damping controls, they're typically located at the bottom of each fork tube.
Adjust the suspension to fit your riding style and weight. If you ride over very tough terrain, you'll want to have heavier damping and longer travel. In addition, the fork should have a certain amount of sag, equal to around 20 percent of the fork's travel. If you have 5 inches of travel, the fork should be compressed by 1 inch while you're sitting on the bike.
Overhaul your fork. This should be done when the fork starts to feel sluggish or creaky or when it blows through its travel too quickly. Remove the front wheel and front brake as well as any screws or clips holding the blades of the fork in the lower tubes. Slowly pull the blades out of the lowers to prevent oil spray.
Clean the insides of the fork carefully, being sure to drain all oil into a measuring cup and replacing it with fresh oil as per your fork manual. Grease any screw heads and tighten all of the screws into their original positions with your fingers. After all screws are in place, firmly secure them without overtightening.