The PS3 system is a very powerful machine. It uses an 8 core processor called the Cell Processor and an RSX GPU chip from NVIDIA. The PS3 is a small system and because of this, heat builds up extremely easily inside the system. There are many different steps to take to keep the heat inside the system low so that the PS3 doesn't over heat.
The first thing to keep in mind is that over heating can cause damage to the system. The CPU and GPU are soldered to the motherboard using a Ball Grid Array, or BGA. A BGA is a ton of small solder balls that sit between the CPU/GPU and the motherboard. If one of the solder balls crack, the chip can no longer communicate with the motherboard and will no longer function.
First thing to do is to make sure the PS3 is not sitting on the floor, and especially not on carpet. The PS3 system acts as a vacuum and will suck up any dust and dirt around it, that dirt and dust can cause the PS3 to run very hot. The best place for the PS3 is to have it waste height and away from any other heat sources.
Second thing you can do is to clean out the PS3 regularly. Using a can of compressed air you can blow air through the different vents in the back and front. Using compressed air will loosed up any dust and dirt that is stuck and when the PS3 is turned on its internal fan will blow out that dust and dirt.
The last thing you can do is to buy an external fan for your PS3. These fans are only around 20 dollars and attach to the back of the PS3. this adds another fan to the back and will help to funnel even more dirt and dust out of your PS3 and it will do it at all times. Additionally an added fan will help pull air through your system at a greater rate which will cool your system more efficiently.
Maintaining your PS3 and keeping it clean and cool will extend the life of your system greatly.