Lose dust can be blown away with compressed air. Pressure packs are available for this purpose at most consumer electronic supply outlets.

Compressed air is especially useful for cleaning the interior of the heatsinks, cooling fans and other areas in the case that are hard to get at.

Clean the difficult-to-reach areas on the tray with cotton buds soaked in isopropanol. Use lint-free cloth, or wet-strength paper towels to wipe the open areas.

I also use eye-glasses cleaner pads to give the tray a finishing shine. The lint-free wet-strength paper that comes in eye-glasses cleaner packs is suitable for other cleaning tasks on the Mac Pro.