Miles Kimball (Michigan) and Noah Smith (a recent Michigan PhD who's at Stony Brook) offer a useful "complete guide to getting into an Economics PhD program." Michigan's Jeff Smith comments on it. I generally concur, though like J. Smith, I have a slight disagreement with their emphasis on being a research assistant - I definitely think it can be useful, but programs aren't going to require applicants to have this experience.
My own advice is here. I haven't updated it in a while, but I don't think much has changed.
When I next revise it, I might add some information from the NSF's survey of earned doctorates. Of 1124 new PhDs from US institutions in 2011, 656 had definite employment, 90 were going on to postgraduate study, 218 were seeking employment and 23 were "other" (apparently not everyone completed that survey question). I think that might give a misleadingly negative impression the job market for economists - while it may be that quite a few PhDs weren't employed when they completed the survey, economics PhDs generally are able to get jobs. Of those with employment, 56.3% were going into academe, 14.9% to government and 16.8% to business, with the rest going to nonprofits (6.6%) and other/unknown (5.5%). The cohort was 34.4% female and 38.2% were US citizens. The median age of a PhD was 31.4 and the median time to completion was 7 years.
The American Economic Association also has some useful resources about Econ PhD programs.