Air Vents
The air vents for the PS3 Slim are located on the back of the device -- unlike the air vents of the regular PS3, which are located on the side. This means you should not place your PS3 Slim up against a wall, as this will block air flow and can cause it to overheat. If you must place it against a wall, feel the device with your hand every hour or so to make sure it's not getting too hot. The cooling fans inside the Slim should help keep it at a reasonable temperature, but overheating can still occur.
Open Spaces
Position your PS3 Slim in an open space, such as on a desktop free from clutter or a table with nothing else around it. Do not place it next to other devices, such as your computer, that also generate heat. Keep a small fan nearby that can help keep the inner workings of the device cool. If possible, have the windows open while you play so that natural cool air is filling the room. If it's hot outside, crank the AC to chill the room. The cooler the environment, the less likely your PS3 Slim will overheat.
Keep It Clean
Use a small vacuum with an attachment to suck out any dust that has collected within the vents of the PS3 Slim. No matter how clean you keep your room, dust will eventually build up, and this dust, if stuck in the vents, can cause overheating. You can purchase a stand that positions the console vertically rather than horizontally. This may help keep more dust out -- as the vents will be more elevated -- but won't keep all dust out.
Take a Break
A logical way of preventing your PS3 Slim from overheating is to let it cool off. Outside of playing video games, the console can be used for online chatting, video watching and Web surfing, all of which will cause the device to heat up. If you have other devices to search the Web or watch videos on, use them instead of your PS3 Slim. When not in use, unplug the console and let it cool for an hour or so. Video games can be quite addictive, but you can always save your game and return to it later after your PS3 Slim has cooled off for a bit.