This may not look like many steps, but there is plenty of information on each one!
Step 1: Heat: A computer's kryptonite
First, I must explain how the computer case aids in the cooling process. Most likely you will notice a vent at the bottom front part of your computer case. Cool air (the air is cool because it is lower; heat rises) is drawn into the computer through that vent by a fan that is right behind it. As the air is sucked in, it cools the components down; but, the air gets hotter as it rises. A fan on the back side of the computer near the top pulls the hot air out. It is also drawn out by the power supply's fan which is right above the rear fan. In some computers, there is no rear fan, just the power supply's fan.
If you place your computer so that something blocks one vent or both, (i.e. a wall, cabinet) then cool air can not come in, or hot air will not be allowed out; both of which results in a hotter computer. Also, if your computer is kept in a confined space, the hot air will be blown into the area and stay there; then, hot air will be sucked back into the computer.
Another thing that keeps your computer hot is dust, believe it or not. Dust is like insulation, or a blanket. The design of your computer case creates a suction effect. This suction effect is also important in cooling your computer; but, just like bad placement can block the vents and make them useless, bad placement can turn a positive design into a negative, turning your computer into a literal vacuum. If you place your computer on the floor, it will suck up all of the dust that lies there. You might not even have to use your Dyson anymore. This is especially bad on carpeted floors because carpets hold on to so much dust...you might even get loose carpet in your computer. One thing you can do to keep your computer off the floor AND off the desk is to purchase a "CPU" holder. (CPU really means processor. For some reason, people call computers CPU's, this is incorrect; hence the air quotes)
Despite your diligent efforts, dust will inevitably creep into your computer. You should regularly clean it out with compressed air. NEVER use a rag or cloth to clean it out. NEVER use cleaning products or other liquids to clean it out. Refer to my Instructable, "GEEK-TO-YOU: How to safely troubleshoot, clean, and repair a computer" for an in-depth guide on cleaning your computer and other important information.
Fun fact: Your computer's processor, or CPU (central processing unit), does so many calculations, and gets so hot, that with out a fan and heat sink, it would burn through the mother board of your computer.