I don't clean things out nearly as often as I should. But I do keep an eye on CPU temperature, so if anything gets seriously clogged I will know about it before it causes CPU cooling issues. My GPUs are passively cooled, and I avoid motherboards with chipset fans like the plague, so they do not collect dust nearly as much as the CPU HSF.
When I do clean, I use a canned compressed gas duster and a vacuum cleaner. I hold the vacuum hose near the area I'm hitting with the compressed gas to reduce the amount of dust that gets spewed into the room. If the system has major dust bunnies, or if I get a really disgusting system
from someone else for a tune-up I will do the initial dusting outdoors.
Don't forget the inside of the PSU! Out of sight doesn't mean it should be out of mind. No, I don't take the PSU apart; what I do is hold the vacuum cleaner hose by the PSU exhaust vent, and aim the nozzle of the compressed gas duster in through the PSU air intake. This creates enough turbulence inside the PSU to dislodge most of the dust.
A side note on the compressed gas dusters -- they're stupid expensive at most places, but if you buy them in bulk at Costco they are like only 1/2 to 1/3 the cost per can. For me that works out to something like a 3 year supply.