You can use cinnamon oil to help plump up the lips. You can find cinnamon oil at any new age-y type store or online. Or you can make a paste of water and cinnamon like you'd have in your kitchen, and apply that to the lips for a few minutes.
If you want to make the lips *appear* fuller using makeup, there are a couple of ways to do that. One is...after you've applied your lipstick or lip stain, add just a small dab of a clear lip gloss to the center of the bottom lip only. This will draw the eye to the fullest part of your lip and give the illusion that it is poutier than it is. If you want to make the top lip appear bolder, use an illuminator in the center, just above your lip on the cupid's bow (the part of your upper lip where it dips down in the middle). This will also create the illusion of a fuller, more protruding upper lip.