- Clean dust/dirt off the motherboard and other parts as you proceed inwards, using the paintbrush and vacuum cleaner on low suction. Using C02 pressure cans to blow the dust off is arguably a waste of time as it tends to redistribute a lot of the dust (along with your money).
Vacuum cleaners suck air and dust, that is true. However, this suction creates a vortex of air and is known to generate static electricity in the process. Compressed air doesn't generate static electricity. A damp cloth for collecting dust particles/fluff etc. placed in the direction of where you are to blow air is just as good. Better is to work in a well vented area or simply do the blowing of air outside.
Oh, to prevent damage to fans, blowing or sucking air can make the fan turn in RPMs it was never designed for. You wouldn't be the first to damage the fan more by cleaning it than the dust/cruft could ever do. So it is necessary to physically stop the fan (or fans) from rotating at all, when cleaning out dust. Slowly rotating the fan(s) when cleaning debris from fan blades is fine. If physical access to fan or fans is possible, use a finger or toothpick to stop the fan from rotating.
Light application of grinded (ground?) graphite (the dark stuff inside a standard pencil) on the rotating body of the fan(s) where friction is most likely to occur also makes the fan turn smoother. If done right. A smooth running fan draws much less power than one that has trouble rotating. You can quickly test the smoothness of rotation by yourself blowing gently into the fan and see how quickly the fan blades stop moving. Less power consumed usually results in cooler running computers.
Heck, graphite also works well when you have a (slightly) stuck key on your keyboard.
Canisters of air are a bit of a rip-off. Personally, I use an air compressor that is normally used for bicycle tires. And then only ultra short bursts. Doing that twice a year for almost 15 years and haven't destroyed a computer yet. But if that sounds daunting, you can buy hand devices that blow air in a similar way like compressed air canisters, except you don't need to constantly buy new canisters of air for it.
- Heat-conductive thermal grease: You probably only really need to tackle the task of cleaning off and replacing the heat-sink thermal grease if the CPU/GPU has been overheating and shutting down the system. I read somewhere that the grease has a 10-year life expectancy. I have tended to replace it only when I have opened up a laptop for the usual full cleaning (as above) and as a just-in-case measure on older laptops. Speedfan metrics will generally be a good guide as to whether this overheating is a problem. Those greases seem quite expensive, but maybe you get what you pay for. I'm not sure.
Better replace the thermal conductive paste every 5 years. That is what I do and still have three 15 year old systems (Linux, GUI-less) in operation, each doing one task. My boss likes the adagio of 'if it ain't broken...'). While most systems use a paste (not grease, never ever grease!), there are also thermal pads that manufacturers use to transfer the heat from the CPU/GPU to their heat-sink, which are in turn cooled by the fan or fans inside your laptop. These pads last longer, but are less efficient than most paste is.
Paste comes in various degrees of heat conductivity. And that is usually the difference between cheap(er) and expensive paste. Visit overclock forums to get an idea of good paste brands. Paste works best when it is lightly applied to the top surface of the chip you are trying to cool. While the surface of a chip that needs cooling might feel like a butter smooth surface to the touch, it really isn't.
Ideally, you'll put just enough to fill the creases of that chip surface with the paste, so the surface of the chip and the heat-sink make 100% contact over the full contact surface. Put more paste, and you'll decrease the capacity of the heat-sink's capacity to collect the generated heat. Less is more, so make sure that you use 3 or 4 little drops of paste in strategic locations on the CPU to get that 100% coverage. Use less drops for smaller chips.
Never ever use both thermal pads and paste! Only thermal paste or only pads. Cannot stress that part enough.