The trick is that it doesn't happen by accident. You may want to use an opt-in form so that parents can give you those addresses and give you permission to email. You'll most likely need a different system (and a separate OK from parents) than what the school itself uses. You'll also want to talk to your principal now so that she knows and is OK with your plan for communicating via email.
Another step is deciding how you'll be emailing, where those email addresses will be stored, etc. While some groups still use one member's email account, that can be very limiting, and better solutions exist. The best example is our new, free Parent Express Email service that manages all the addresses and helps you send good-looking emails quickly and easily. It's definitely worth a look for the new year.
Simple as it is, I consider email the PTO "killer app" of 2009-10. It's definitely time to get on board.