Here's a downloadable version of my guide for UCSB students thinking about continuing on to a PhD in economics. Or here's some advice from Greg Mankiw and some advice from Susan Athey. Miles Kimball and Noah Smith have written "The complete guide to getting into an economics program." Noah Smith has also written on why you might want an economics PhD. The Summer 2014 issue of the CSWEP newsletter also has a series of fantastic articles.