If you use for a longer period of time, don't idle PS3. Turn it off and turn it back on later (even if it's like 5-10 minutes).
If you're really that worried about it, after long gaming sessions, it would be a good idea to exit the game but leave it running for a few minutes. Sitting at the main menu doesn't work it hard at all, but leaves the fans running. Turning it off leaves it to bake until it cools down on its own.
When playing Uncharted 2, my PS3 would get really hot, and the fan would kick on high. Even with it just sitting on the main menu afterwards, it would take a good 10 minutes or so for it to cool down enough for the fan to slow down.
This is one of the best things you can do if you're a marathon-gamer like myself. I don't have a PS3, but I bum play time off of a buddy all the time. If the thing gets too hot, we'll leave it at the main menu. Sometimes it can take like 15-20 minutes for the fan to slow down if we've just come off of a huge marathon. With my 360, it's kind of the same thing. I like to leave it at the dashboard for a minute before turning it off.
As for dust, I support the compressed air idea. Leaving it upright can help, but it also puts it in danger of falling if you're a clumsy person or you live in an active household. Beware of children. Keep it in an open, dust free environment. If you see dust [/i]on[/i] the system, that means there's most likely dust
in the system, so keep that in mind. USB Laptop coolers and box-fans are your friends.
Lastly, there are adapters to buy for PS/PS2 memory cards, but they don't seem to work very well. My friend and I screwed around with his for hours and couldn't get it to read the card for some reason. Maybe you'll have better luck, but I wouldn't put my faith in it completely.