Don't sit and press the "Check Mail" switch like a lab rat hoping to get a tasty food pellet.
Yes, okay, you might get something you can answer quickly and scratch off your list, but that will not be as important as what is currently on the top of your to-do list.
Set your automatic mail check to every six hours. Yes, I said 6 HOURS.
As you get done with the things you've prioritized as important for today, you will check for new mail, but not before you've gotten at least the #1 thing on your list done.
As Merlin Mann said
– "okay to check first thing in the morning to catch changed priorities"
Set the timer for this one, though; you are only processing. After you go through it, you will do the most important thing you've labeled "Urgent". I like to download the mail at home and then process it on public transit.
– "close mail program when doing tasks that don't require something in email"
Cut out the distraction. At the very least, minimize the program and hide your active programs Dock or toolbar.
– "buy microbreaks too"
Want to spend 5 minutes checking personal mail? Not 'til you've crossed something off the list, bucko.