Should I keep my 360, standing up or siting down flat?

It really doesnt matter, ive had mine since launch. Ive had it sitting both ways and it broke down on me each time. It sitting up right now have been for over a year and its been fine.

flat cause my crazy cousins

kept knocking it over

Vertical, I have a chill out station for my 360 which constantly keeps the console cool and I have to place the 360 vertically to connect to the station.

Also having the chill out station ensures that it's secure vertically, and there's little chance of it tipping over.