* If there's a question for the person to answer, make sure it is the very last sentence in the email. That way when the person finishes reading the email and is deciding whether to ignore/archive or reply, they're more likely to reply. If possible, make the question yes/no instead of long answer. For example: "Is now a good time to move forward?" versus "How does this sound to you?"
* Keep the email short. People generally don't like reading long emails, and even if they start they're less likely to finish it if it's long.
* If you've been following up with someone for a while, start off an email with "I wanted to follow up with you on this one last time". This reminds the person that you've emailed several times before (and perhaps makes them feel guilty for not replying sooner) and lets them know the ball is completely in their court. I have found this to be very useful in kick-starting leads that went stale.