(I'm assuming you're a Windows XP user running Outlook, so Mac/Linux users maybe you can do this in two or one steps?)
1. Click Start > Control Panel > Sounds and Audio Devices.
2. On the Sounds tab, scroll to New Mail Notification in the list of program events.
3. Change the Sound to be played option to "(None)."
Congratulations, you've broken your Pavlovian chains and are free to concentrate on your project tasks without the need to check email every time you hear a ding.
In future posts, I will demonstrate how to logout of Facebook, set your cell phone's ring mode to Silent, disable your instant messenger alerts, and turn off your Twitter client.
UPDATE: Apparently in Outlook 2007 you're better off going to Tools > Options > Email Options > Advanced Email Options, and unchecking all the items under "When new items arrive in my Inbox". You really have to focus to eliminate these distractions.