The classic answer to this question is that you can make your thin lips look fuller by outlining them ever so slightly in a subtle but definite wear proof lip liner, then fill in with lipstick. Some experts suggest also applying a little pearlescent or shimmery lipstick or gloss in the middle of your lips after the full coat of lipstick. There also several "lip pumper" glosses and lipsticks that will temporarily enlarge the size of your lips found at Sephora and other beauty supply stores.
The post-modern answer is that you can get down to the cosmetic surgeon and have him or her plump up your lips with a little collagen injection, which will last for months. If you don't care for needles but just want to plump up your look a bit, you can also use a colored lip gloss to make your lips look fuller. Try a slightly darker shade than you normally wear.
