Once in the city, do yourself a favor and follow @mbta_alerts on twitter.
Its an unofficial, but a good and widely used tool tweeting out every timely service event across the system.
If you want something more official and customized, head over to http://www.mbta.com/rider_tools/t_alerts/. There you can customize what lines you want to get updates on. You can also receive alerts via email or sms. You will have to submit an email or phone number for this service.