There are several email reminder services out there, but I have found Nudgemail to be one of the most useful.
If I need to action an email at a future date I forward it to Nudgemail with the date I want to see it and off it goes, out of my inbox. If I want a diary reminder, I send an email with the date and details to Nudgemail and it comes back when I need the reminder.
I'm using it for work, personal things like insurance renewal reminders, bill payments, prayer follow up with friends. It's currently free in beta, take it for a spin.