Rewards always work better than punishment, this has been proven in pet training. Chocolate would be good because it causes the brain to release Dopamine - the pleasure hormone - therefor making you feel better. We want a positive association with LDs, right? Otherwise you might end up feeling stressed before bed (because you might not LD and therefor get punished), and fear is not what we're looking for, right? With rewards, you'll go to bed thinking "Yay, if I dream Lucid tonight, I'll get a yummy!"

..of course, this is just my personal opinion.