Speaking from what I've learned from my friends, one of the best ways to make your lips look fuller is to use a good lipbrush when applying lipcolor to your lips. Basically, if you feel your lips are thin:
1) Apply lip liner just slightly outside the edges of your lips
2) Fill lips lightly with lip liner
3) Take lip and lipstick. Do not apply lipstick directly to lips. Use lipbrush to put color on your lips.
4) Use lipbrush to get the color from the lipstick, then apply color on lips in vertical strokes to both top lip and bottom lip.
For a fuller look, use light, glossy colors. Don't use dark matte colors. Good luck! And try a google search for lipstick application tips for more info.