I would have to second Robyne on the Juvederm or Juvederm Ultra Plus. Both of those are thicker than the comparable versions of Restylane/Perlane and should last longer.
The over-injecting is the main cause, but the second is where it's being injected. Usually the lip injections along the border will look fake. When the fat part of the lip is shot up it looks much more natural IMO.