Here it is...directly from Nikon's(I mean horses) mouth! If you are a casual photographer who shoots less than 10,000 shots a year, simply bring your Nikon D90 in for an inspection annually. Always request Nikon Service Center to clean your body mirror, remove all accumulated dust inside the body with their special air blower machine and recalibrate your kit lens (together with any other lens you may own of Nikon's).
Specific parts that are subjected to attentive cleaning include the viewfinder, lens mount assembly, mirror, SD card slot compartment and even contact points of buttons within the body. Remember, our Nikon D90 is not weather-sealed!
For the uninitiated who buys their Nikon D90 without a proper electric dry cabinet, please also ask them to check for possible fungus growth and help eradicate them. They may or may not charge for this. I know Nikon charges for doing that for lens that are known to be fungus attacked.
Now, if you are shooting at over 20,000 shots per year, you may want to do the above every 6 months! After a holiday or extensive shootout, you may want to do a little self maintenance with cotton buds and lint-free cleaning cloth. Most importantly, drop your camera back into the electric dry cabinet to dehumidify the entire system first. My experience on this is that humidity index takes over a day to drop back to the optimal level (preset in my cabinet) whenever I drop my camera in after prolonged exposure to outdoor! That means its very "wet".
In a way, treat your camera like your family sedan...