One strategy that I'm really enjoying at a current gig: have the person writing the front-end write the end-to-end tests (or at least the specs) for the person writing the back-end. Just having a fresh set of eyes on the tests has flushed out a lot of edge cases, and tests make a perfect place to iron out small behavioral details. It really solves the "same mind writing the code and tests" issue.

Of course, this isn't always feasible (and is a little unfair to the frontend person in terms of workload).