There aren't many good writer/director teams that come to mind where they have worked multiple times. I guess you could make an argument for Alexander Payne and Jim Taylor, buy Payne directs as well. Then there's Robert Bolt and David Lean. Can't think of too many other films I love where the director isn't one of the writers.

Oh, forgot a big one. Paul Schrader and Martin Scorsese, of course.

Michael Mann and Eric Roth have a great rapport, though not a lot of films behind them at the moment. Hopefully they get their western off the ground in due time.

I also like the collaboration of Arriaga and Inarritu, when they aren't at each others' throats. Seems to be we'll only have three films from that duo.

Chris Nolan also works very well with his brother.

Billy Wilder and I.A.L. Diamond is a good one or Billy Wilder (as the writer) and Ernst Lubitsch.

Robert Towne and Warren Beatty had a good partnership for a while, but that was writer and actor, not director. I suppose you could say Towne and Hal Ashby, since they did two films together.

I'd need to think about this further...

Fran/Phillipa and Peter