i have a very thin top lip that disappears when I smile. What's the best procedure to make it look thicker?

you may be a good candidate for a lip lift procedure and/or filler injections in and around the lip/nose/mouth.  I suggest seeking the advice of a dermatologic surgeon who performs both these procedures.  Best, Dr. Emer.

The best option would be to add a filler such as Juvederm or Belotero to the lips.  A more accurate recommendation can be made, of course, during an in-person consultation.  The attached link provides more information.I hope this helps.