If you're writing desktop applications, simply following the UI guidelines for your chosen platform will take you a long way.
If it's on the web then you're broadly screwed, you just need a designer.
That said, don't get fooled into thinking that UI design is all about the the visual appearance. Having the right interaction model is probably more important. A graphic designer isn't going to help you with that. If you don't have access to a UI specialist then try starting with User Interface Design for Programmers.
