While I don't own an Xbox 360.. Pretty much all rules apply.
Keep your unit in a well ventilated area. Airflow is the most important part.
Keep your unit free of dust. Once or twice a week, dust the unit with a microfibre cloth.
I myself prefer to vacum the vents in my PS3, as opposed to using a can of air duster. If you have a Dyson vacum cleaner, or anything similar, use a small attachment to vacum out the vents every month or so.(depending on the dust content of your living space)
That's pretty much it.