Lick/smack your lips
Definition 1 of 2
Pro Tip 1/3Hungry Anticipation
Use when someone can't wait to eat and shows it by preparing their mouth. 
Seeing the cupcakes come out of the oven, the kids licked their lips.
Pro Tip 2/3Metaphoric Hunger
Considered more playful or expressive than the direct statement of hunger or desire. 
He licked his lips at the prospect of the pizza buffet.
Pro Tip 3/3Non-verbal Cue
Often accompanies a physical gesture that suggests hunger or craving. 
She licked her lips hungrily, eyeing the last piece of cake.