Merlin over at 43 Folders always has useful advice, but this one on Writing Sensible Email Messages is essential reading:

Before you type anything into a new message, have explicit answers for two questions:

1. Why am I writing this?

2. What exactly do I want the result of this message to be?

If you can’t succinctly state these answers, you might want to hold off on sending your message until you can. People get dozens, hundreds, even thousands of emails each day, so it’s only natural for them to gravitate toward the messages that are well thought-out and that clearly respect their time and attention. Careless emails do not invite careful responses.

I get an awful lot of emails myself, and I wish more people considered making them easier to read… and to respond to.