Vacuum
A vacuum with a hose & brush attachment works for me. The brush attachment keeps the suction from damaging stuff by dispersing it through the bristles. Always power off first naturally and unplug. Open the case and in a minute one can have a pretty good clean even around the cables an hard to dust areas.. I try and hover over and lightly brush the mother board and other sensitive areas.
I find that works pretty great for the key board too. Hair, dust and 'floozles' get dislodged pretty well.