I have a reward system based on points that I accumulate daily, lucid dream or not: DJ = 1, 2 or 3 points (depending on the number of recalled dreams), frequent RC during the day = 1 point, LD = 10 points, etc.
Then, I use the points as an "authorization" to go to the movies with my wife (1 point = 10 cents to spend). The idea is to reward the behaviors I know are good for lucid dreaming, even when I have long periods with no lucid dream.
I also keep telling myself that everything I do to increase my probabilities of LD also improve my memory, which is a good thing by itself.