This research showed that female servers who drew a smiley face on the check increased their tips. Drawing will personalize you to the guest and increase their empathy for you as a person and may communicate to the guest that you were happy to have served them.
On the other hand this backfired for the male servers, decreasing the tips they received when drawing a smiley face. Now guys if you can draw, use a cartoon character, this may help increase your tips.
Start doodling and see what you come up with.