I had two back to back lucid dreams for the first time, and in both, the first thing i did was run to the wall and play with the light switches. When nothing happened, I knew I was lucid. It was so easy, and happened at the very beginning of the dream, within seconds.

A friend told me to use this technique, and I just had my first 2 LDs because of it.

He also said he tries to scream in his dreams, and when he cant scream or it comes out as regular volume speech, he knows hes lucid.

Cheers.