Obviously this isn't an absolute, and there are exceptions to this rule. The most important thing I can get across is this; End users do not, and will not, think like you do about your software (Read 10x). The reason you must get perspective outside your own is because yours is tainted beyond repair when looking at your own project.
So, what can you do to make your open source project more appealing to a designer?
1) Let go of preconceived bias when it comes to your UI / look-and-feel.
Someone more interested in the visual appearance and usability of your application is going to want to make changes to the workflow. Just as you test your code, test your design to make sure the designer's assumptions hold up. As a president once said, "Trust but verify."
2) Develop a clear understanding of where your designers implementation responsibilities end and yours begin (and vice versa).
Which is to say, know their limits, and know your own. If they don't know how to integrate their work into your project then either help them or find a third party that can. If you don't know how to integrate their work into your project then have them help or find a third party that can. Focus on your strengths. Nothing is more dissatisfying that seeing your work butchered.
3) Check the ego at the door/keyboard/whatever.
This applies to everyone involved in any way, shape, or form. Objectivity is the ideal, but lets be realistic. Personally I know more people who have had bad experiences volunteering / interacting with open sources projects than good (including myself). While this is anecdotal I think it's still important. Don't mistake popularity with success, and don't let popularity become validation for your lack of social skills. The people at the other end of your emails, IMs, IRC channels, etc are just that... people. The golden rule gets turned up to 11 when someone decides to take some of their free time and donate it to your cause.
This is hardly comprehensive, but I think it touches on a few points in ways I have yet to see in this thread. Keep doing your good work.