If you've already had one lucid dream, then you have the benefit of knowing what your goal is. Like Seeker28 said, be sure to check out the Tutorial section.

My favorite way to stay in the dream is to look at my hands, rub them together, and run them over an object close to me. Touching things and feeling the texture of solid objects (even the floor) helps me stay grounded in the dream. I also like to have a simple goal set for myself ahead of time. That way, I can immediately know what I want to do