A mountain bike's suspension is one of the most important components on a bike. It is also one of the most expensive. Yet it is probably the most neglected. This is due to the complexity of most newer forks, the cost of service, and an inability to tell when service is needed.
Suspension forks consist of two parts: a spring and a damper. The spring is usually a big air chamber whichprovides most of the resistance you feel when you press down on your handlebars. There is also a negative spring which is sometimes a smaller air chamber and sometimes an actual spring. The negative spring reduces stiction at the beginning of the stroke and acts as a bumper when the fork returns to full extension.
The damper is the more complicated side of the fork. Most forks have an oil damper, where a fairly large volume of oil will flow through a bunch of little holes giving you the ability to adjust your fork's rebound speed and lock out your fork while you are riding. More advanced dampers will also let you adjust your compression and lock out your fork with a threshold, where your fork will blow past the lock if you hit something with enough force.
Every part of a suspension fork needs some kind of service. Most obviously, seals and o-rings need replaced. The two big seals between the lowers and the stanctions take a lot of abuse and will eventually start to let dirt into your fork. And all the o-rings on the inside simply wear. Just as important, however, is that the oil needs replaced. Suspension oil actually heats up while you ride, and after a while its viscosity changes and it just doesn't work as well anymore.
The problem is that changes in the performance of your suspension fork happen so slowly that you don't notice them. And when your fork isn't working right, it doesn't keep you from riding the way a worn out drivetrain will. You aren't constantly reminded something's wrong. But failure to keep your fork maintained can end up meaning costly repairs down the line. Stanchions can wear if the seals get too dry. Dampers can break if the oil gets too thick. Before you know it, it'll be cheaper to buy a new fork.
Your best bet is to stick to the manufacturer's recommended service intervals. SRAM says complete service should be done every 100 hours of riding, but with riding conditions in the PNW more frequent service is usually required. So do the math. If you ride twice a week, two hours each ride, that's 16 hours a month. Then say you do one four hour ride on a weekend. That would mean you should probably get you fork serviced every five months. So if you generally ride in good conditions and ride less in the winter, you should definitely get it done annually.
Full service on a suspension fork around $120, and we generally keep seal kits in stock for all the major models of Fox and Rock Shox forks and rear shocks.