I stabilized myself in a recent lucid dream by punching the surfaces of the inside of the car I was in. I just started smacking everything within reach, the pain was minimal, and the tensing of my muscles got the car moving, even though there was no driver.

I think the reason you wake up so soon is an overload of excitement, as other important DV members have said. A little is just enough to keep you from slipping back to non-lucid, too much will make you wake up. After a few lucid dreams, you should be able to remind yourself mid-dream to "stay calm" and "focus" and things like this. These have worked for me.