In reply to The Potato:
At the risk of getting a bit of flak, I suspect that lots of people have too much damping on their forks. Also manufacturers have something of a tradition of allowing an adjustment range way in excess of what is actually needed, e.g. on some forks if you wind the rebound all the way shut the forks will barely move. Some companies avoid this, realising that it's unnecessary. Of course it may well just be bad design.
I tend to run both compression and rebound as open as I can get away with, e.g. HSC all the way open and only a few clicks from open on LSC and rebound. The less damping, the better the fork can track the terrain and maintain contact and hence traction with the front wheel. Obviously if you're riding at pro level then you'll want it to be a lot stiffer as riding at higher speed generates bigger forces. It's not unusual for people to use the damper to compensate for a poor performing or poorly set up spring as well.
Why do you feel it's too quick currently?
Nerdy discussion of mtb suspension aside, if you want to slow it down you have two options really:
Easiest thing is to change the damper oil for a heavier weight. The service manual for the forks should tell you how to drain and replace the oil and what type/weight to use. Just for clarity, this is the oil in the damping cartridge, not the lubrication oil in the lowers. Be aware that there is little correlation between oil weights and actual damping properties between oil brands. Also not all oils are compatible with all seals. Safest way is to find the recommended oil type and get the same one but in a heavier weight. The mtbr.com forums tend to be full of info on this sort of thing if you really want to get nerdy about it.
Your other option is to change some of the internal parts of the rebound assembly (this is generally what people mean when they say 'tune' it). Better dampers tend to use a shim stack to control oil flow through the damping ports, by changing the sizes and orders of the shims you can change the damping properties. This is not necessarily an intuitive process, but there's plenty of info out there if you want to try it. If it doesn't use a shim stack in the rebound circuit then you'd have to do a bit of research to find if anyone makes aftermarket parts to allow you to tune it. A reputable suspension tuning company should be able to do this. Make sure it's a reputable company though. It may be cheaper/easier to change the entire cartridge for a better one though.
This can be quite an involved topic so apologies if any of the above needs a better explanation.
