My OB said there was a study long ago that showed that it was best to wait at least 3 months. He said in recent years it has been discredited (and he said it wasn't a good study to begin with), but that lots of doctors still follow this advice. Obviously, reproductive health is a difficult thing to do a controlled study on so there are very few clear answers. I believe there was a more recent study that showed women had a better chance of a successful pregnancy if they did get pregnant again within 3 months of miscarriage, but again I don't know how well controlled it was.
All that said, my OB said to wait one cycle (mainly for dating), but that we could try again right away after that as long as we felt ready. I think a lot of doctors recommend waiting because they worry about women thinking they are ready then really struggling with a new pregnancy so they suggest they wait.