There are of course several options for lip enhancement.  Overall though, fillers are the best option.  They are temporary, and fillers such as Juvederm, Restylane and Perlane are excellent options that can provide a natural looking and feeling result.

A more permanent solution is fat grafting, but this should only be done by a highly skilled and experienced surgeon.  Lip augmentation is a bit of an art form, and facial aesthetics should always be taken into consideration as well.  Do your research and find a reputable surgeon no matter which option you choose.