Some very useful writing resources for PhD students are available at John Cochrane's University of Chicago Writing Group.
Not just about writing, Hal Varian's paper on model building has some useful advice, including about forming research ideas.
There is also some material from the finance community on Youtube:
A recent paper in the Journal of Economic Perspectives by Berk, Harvey and Hirshleifer provides helpful advice on writing referee reports.
If you are not sure which word to use, Tim Martens alerted me to ngrams, which is very useful for assessing prior usage.