Something to consider: pure coders tend to be more receptive to guidance in their craft than interface designers. No decent coder wants his/her app running with bugs. They make it look bad.

Artists tend to think that their work is the best approach, so they're not as receptive to constructive criticism. They don't think in terms of "bugs". How can their art be flawed?

A coder also performing as artist/designer without the proper experience/skillset there is inclined to first take a utilitarian approach and fail when trying to incorporate clever graphic elements. They will have to be sold on the idea of someone else coming in to revamp that part of the user experience.

The best sales approach IMO is to first convince the developer he/she can better maintain the code logic by divorcing it from the UI. The next step is to propose yourself as the person to enhance and maintain the UI as a team member.

Not every coder will be receptive, but those with the users in mind will. We have examples in Dialcentral, eCoach, and others.

Developers who refuse to consider a "slick" UI will watch as competitors bury them in downloads.

EDIT: like Jsuggs said above.

disclaimer: I am a coder and artist.