Digital clocks. Digital Clocks. And more digital clocks. When I was hardcore LDing, I had 2 in my room, visible from any angle. They paid off in some epic quality false awakenings turned LDs.

Seriously, nothing else comes even close to the reliability of a digital clock RC. I've tried finger through palm and breathing through the nose, but they do not produce the same "jolt of awareness" that you feel as you are looking at a changing clock. "WTF is going on with this clock?" .