I'm not sure whether this would require too much thought, but I have a subtle reality check that I like. I try to recall how I got to where I am and demand proof that it makes any sense.

It's quick, very subtle, and immediately engages your critical thinking (which is great for becoming lucid.) The one downside is that you do have to think for just a moment. If I had any singing talent, I could try to tell you whether or not this is a problem.

Good luck! And that's an awesome dream sign, by the way. LD singing is even better than shower singing.