The best tip to give a computer user is to clean the dust out of your computer regularly, if your computer is in a dusty environment, the fans that cool the CPU and GPU will gradually fill with dust until the efficiency of the heatsink and the fans is reduced. Switching off your computer and cleaning the fans and heatsinks is a good idea, this will extend the lifespan of your components as overheating will damage the components. The two pictures on this page show a couple of cases where a computer has gotten far too dusty, you should never let your computer get to this state. Regular cleaning of your computer will keep dust and pet hair out of your precious components, if it builds up it will insulate the heat-sinks and hold in the heat. That is a BAD thing indeed. Like rugging up with a warm blanket in Winter. The heat can cause damage in many ways, with overheating shortening the lifespan of your CPU, and even a fire risk.
The main message of this posting is that taking care of your computer is important if you want your components to last, if you just leave it on 24/7 and never shutdown the computer and clean out the CPU fan and GPU fan if you have a video card, then you will have your computer shutting down soon after booting due to the CPU overhheating and the safety countermeasures shutting it down straight afterwards. I have had that happen and it turned out that I had not secured the CPU properly, but that is another part of caring for your machine, ensuring that the CPU is properly seated when you are installing it. And ensure that the proper amount of heat-sink grease is applied, too much will actually insulate the CPU and prevent the easy transfer of heat from the CPU to the heat-sink to be sucked away by the cooling of the fan or water cooling or even liquid Helium cooling if you have the equipment. But you need to make sure that the heat can get to the heat-sink easily, that is why you need to ensure that your machine is kept cool and the dust is not allowed to build up.
Another link to back up this message: http://hardware.slashdot.org/story/02/07/01/2214239/is-your-computer-a-fire-hazard-waiting-to-happen. [Slashdot.org]. Useful video below showing how to clean out your computer properly.
And how to apply heat-sink grease like a pro.