Vented area, don't move it while it's on, put it on a stable surface, don't block the vents, don't leave it on for very long times(it's worse to play for long times, as the 360 has to constantly read the disc), if you must play something for a long time for whatever reason, install it onto your HDD so your 360 has to do less.
Be warned, though, RROD warranties are about to start ending, as it's been three years since the 360 launch, so make sure to call it in before your warranty dies, if it's an older model.