I actually really hate being that "Hey, didja read that E-mail I sent?" guy, but if you're not responsive, I am going to have to pay you a visit. And I have no idea when you're in the zone and when you're just chilling. If you would just answer your E-mail, then you get to do it at the best time for you.
One thing I've found helpful over the years is: When you are depending on getting a response, say so. "NEED RESPONSE BY EOD TUESDAY" Make it the very first thing in the body of the E-mail, or even make it the subject of the E-mail, since some people only skim the subjects.
Another tip is to state clearly what assumption you will make about their thoughts if you do not hear back in time: "If I don't hear back by EOD, we will go forward with the proposal."
EDIT: Another good tip I forgot to mention: If your E-mail is going to multiple people, and you need one or more of them to respond or take action, type their Full Name in bold and red then ":" then the specific action the need to take and by when. This helps people who only scan their E-mails, as their name stands out.
Most of these tips are actually pretty obvious, it seems silly to even write them down.