My personal favorite is a pretty simple one. It goes, "I will be lucid and aware", "I will remember my dreams", and thinking those two phrases alternately as you breath. One phrase is said on the inhale, the other on the exhale. I say it in my head of course but I make sure to hear my voice in my head.

Finding what works for you in the important thing. It is something that will make you remember, so whatever that is works. You can say "I will dream about (insert location) tonight"..."when I dream, I will eat fudge brownies on Mars"....Whatever engenders that feeling of intention for future action. Mantras are assistants in creating feeling. The feeling I go for is determination to do future actions. In this case to lucid dream.

When you are in line to buy a tasty treat, there is anticipation for whatever is coming. There is a determined future outcome in your mind and that is why you are standing in this line. There is an intention to purchase a tasty treat. That feeling is important for mantras...anyway, hope that makes sense.

Always be positive. Before sleep, when you wake up, always take some time to feel positive. This will increase your recall, lucidity, and improve dream experience. That simple augmentation of emotional orientation can color the entire days activities as well as nights activities.