I would start with the pressures recommended for your weight (some RS forks have them printed permanently on the sliders, or refer to the manual). If you bottom-out every once in a while (on big hits) some say this is about right. I prefer to add another 10psi until it no longer bottoms-out - that way I know I'm not far from the optimum and I get to use all the travel that I paid for. If you've never bottomed your fork, take the pressure down 5 or 10psi at a time until you do. Then bring it back up by 10psi for head room. Once you know what pressure that is, write it down somewhere handy (tape on your fork pump or under your helmet). Your pressure rating could change in the future, as your skill levels and aggression increase (higher jumps, deeper drops) or as you get older like me and put on extra weight (potato chips, beer).

The negative pressure makes the beginning of the stroke nice and plush, and you can use that to fine-tune how much sag you want. Best to stay within the recommended pressure range for your weight. The higher the pressure, the more the sag, the plusher it is at the start... which can be a good or a bad thing depending on what sort of riding you do. Coil spring forks don't have negative pressure chambers, but have spring pre-load adjusters instead (different story, similar effect). You need a bit of sag so that your fork (and wheel) can first extend "into" dips in the terrain rather than the whole front-end just falling into them.

Rebound dampening "slows" down the return action of the fork, so you don't bounce straight back into the air like in a trampoline (think pogo stick, or human catapult). You don't want too much though, best to go for the least allowable. Like chelboed said earlier, the forks will pack-up in the rapid stuff (washboards) if you've got too much rebound damping. It's like the fork's having a brain-freeze cuz it can't react fast enough to what's happening beneath it.

Generally you want the positive air pressure (or spring firmness on coil forks) to handle ALL of the compression duties. Use the compression damper if you want to "slow down" the upward movement or lock it up completely for climbing or hammering in the flats. It's not a good idea to use the compression damper to compensate for an under-sprung fork. I mean it will work, but in some designs it will overheat the fork oil too much and could cause your compression AND rebound to fade (if the circuit design shares the same oil reservoir). In very extreme instances, using the compression damper as an added "spring," or simply when it is over-worked, can froth up your fork oil. High performance systems are sealed with pressurized nitrogen to prevent this.

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