I really wish I knew how to get great designers... VLC is a product, that is widely used, usable (not the mac version, though), but it is extremely ugly. It is quite hard to get designers to help us (redesigning all the buttons for example) for quite many reasons. The biggest reason is that we don't speak the same language. Many designers don't understand the criticism that usually comes around with each modification…
So as harsh as it may sound the answer to your question is that you just cannot get great designers. There's virtually nothing you can offer them in return. If you do manage to get someone on board, do shield them from the "community". Work one on one and put all your weight behind his designs when they are made public. Leet hackorz that are so vocal on many O/S forums and mailing lists can easily demotivate even Mother Teresa in a matter of few replies.
HTH, feel free to vote me down now :)
(edit) On a second thought VLC might be one of few projects that can attract a designer because of its high-visibility nature. Yet the point remains - designers are easily demotivated, way easier than developers, so retaining them is a major challenge.