well, we don't know for sure exactly what the YLOD is, but it generally seems to be an overheating issue like the RROD.
here is a guide to fixing it if you have nothing left to lose.
http://playstationlifestyle.net/forums/showthread.php?t=2376
As for preventing future problems just take general steps like you would for any electronic device.
1) Turn it off when not in use
2) keep it in a well ventilated cool area. If air can't flow around it, then it will slowly bake itself to death.
3) keep it in a clean area. dust from cigarettes or wood fires will kill the thing.
4) clean it on occasion. vacuum out the vents as dust will block air flow which could cause the system to overheat.