Mattan Griffel wrote a great article, pretty much redefining email etiquette for our busy times.
In a nutshell, his five rules are:
- Keep it short
- Format for readability and clarity
- Make it clear what you want me to do
- Be reasonable with your request
- Show me why I should take the time to help you
Read what all this means on his article, How to get a busy person to respond to your email