The ideal upper lip shape has a gentle rise on either side with the highest point being cupid's bow ( the central upper lip where it comes to two points with a depression in the center).
Most physicians often fill the lips with filler predominantly in the the vermilion which is the white red junction; this can lead to a distinct often unnatural lip border. The ideal lip should have definition there but most of the filling should be in the red mucosal portion in order to bring out the lips. Too much filling in the center can give the appearance of a "sausage lip".
For the lower lip, ideally the bulk of the filling should be in the central lower red aspect to create a slightly rolled appearance with much less placed along the vermilion and along the outsides of the lower lip. Again, too much placed uniformly in the lower lip will give the appearance of an unnatural sausage.
Finally, the upper lip should be approximately 2/3 the size of the upper lip and be one to two mm further out than the lower lip. Again too much augmentation there can lead to an unnatural "duck-like" appearance.
In conclusion, less is often better since additional augmentation can be added later on if necessary. Good options include juvederm or restylane. Fat in my opinion is less desirable since it doesn't last a whole lot longer and is more difficult to shape.
