At Digg, we had a couple different ways. As a developer, the most productive time was when the designers would give us PSD mocks that were actually somewhat built from our existing UI widgets (they still had all the freedom they needed but working off of existing designs ensured consistency and some useful limits). Moreover, I know Tyson was concerned about how text rendering differed between PS and browsers so he built a small browser tool to grab accurate screenshots for text.

The best thing was however sitting literally next to the designers during development which made it essentially frictionless to iterate on a design; whenever there was something ambiguous or that wasn't fully explored in the designs, we could ask the designers.