Well, if death was the only option, I suppose the most...assured manner would be to stick my tongue between my teeth and slam my chin down on the floor or wall. If I was...lucky...I'd sever it and bleed to death. Horrible way to go, but if that was the only option, it beats starving to death or trying to pillow fight yourself to death. The backflipping idea would be faster but you're more likely to break other bones (shoulders, arms, upper back, ribs, skull, jaw, etc) and even if you could break your neck there's no guarantee it'd kill you. It might just paralyze you instead. At least bleeding out...slow though it is...is a sure thing.