The advice above is great if you have a real passion and talent for design or if you really need to save the $$$. However, as a counterpoint I'll share some compelling reasons to hire/borrow a real designer, even if that means paying some $$$ for a short-term contract:
(1) It takes time to learn design skills. Can you really afford the opportunity cost? Wouldn't that time be better spent on business/hacker work?
(2) For a credible product, you need a professional finish. Sure, you can go for a simple look like Google but even doing that well takes serious skills. Design is like DIY in your home - it's easy to 'get by' but hard to do a real professional job.
(3) You will probably take longer to do this work than a professional. This means that not only are you spending valuable business/hacker time but you're spending even more time than a professional would.
One problem I see with hiring a designer is that it becomes hard to change the site after the deed is done. It is difficult to make incremental improvements.
It's a good point. You can mitigate this by asking your designer to leave a PSD file with various UI components and teach you the basics of how, for example, to create new buttons in the site style. Of course, this doesn't help with brand-new innovative features or artwork in the same style. Artwork is where you feel this problem at its worst!
