Ah see... you were thinking about that dream with the two snakes, and you dreamed about them again. This demonstrates that we dream about what we're thinking about. It shows that you have the power to influence your dreams. Just keep at it... a lot of people have a lucid the first night after they find out about it, or real soon after - those are "beginner's luck". Then it usually takes a month or two of concerted effort to start to have them more regularly or to be able to incubate them whenever you want.

Definitely get Exploring the World of Lucid Dreaming as Mario recommended. And keep reading the tutorials in here and re-reading them now and then... read about other people's experiences and attempts... anything you can do to keep lucidity central in your mind. Keep a dream journal... this is key.

It will happen, trust me.