what 666sheep said is the true sure way to clean back to brand new. blowing canned air at the heat sinks won't do anything but blow the dust off the top of the heatsinks. Find a take apart guide for your specific model, someone here should be able to help you find that, if not PM me.
The powersupply collects an assload of dust, as well as the squirelcage fan that pulls the heat off the backside of the logicboard thats in the back by your hdd. The best way is to compeletly disassemble it and clean it piece by piece, its not really that hard at all 3 main parts (logicboard, cpus, powersupply) and the fans. I have and AntiStatic brush that I have from photography for lenses and it works great with brushing off the dust on the logic boards or boards in general where air won't take it off. After completely cleaning out mine the temps are a few degrees cooler as well since its getting clean fresh air not dust ridden air with carpet fibres and what ever other kind of particles in the air. I do it every 6-8mo just to keep things like they originally were.