i've worked with a lot of designers, and one that worked out great for me actually contacted me out of the blue, but usually i like to see a track record. a good portfolio is nice, because the portfolio page itself also shows the kind of work the designer does :)
of course, i've worked with webinc based on portfolio but also reputation. and i've also hired designers for projects based on contest entries on gfy.
so - your turn. what makes you choose a designer?