Excellent stuff, by Elad Gil:
Unfortunately, a lot of otherwise savvy entrepreneurs don't follow up with investors well. You have to remember that every thing you do can signal to an investor a lack of urgency/interest in your company, the fact that you are taking your startup casually, desperation, or a lack of ability to follow through. Also, if you don't create urgency or a sense that the investor may miss out on something interesting, then the angel may drag their feet in meeting with you, extending the time of your fundraise.
Elad proceeds to give a solid example of a good investor reply. Bookmark and keep in mind for the next time you need it.