One mistake I have seen is not being clear as to what you want, or limiting yourself unnecessarily. For example, if you want all emails J. Doe sent over the last month. A request asking for emails sent from J. Doe’s email account might not (an will likely not be construed by the responding agency) to include emails sent by J. Doe from email accounts accessible by J. Doe. It is always best to say what you want simply but comprehensively…”I want all of the emails J. Does has sent from any and all email accounts during the last 30 days”.