The Concord Coach bus has hourly departures from Boston South Station to Portland. That is the most convenient option. No MBTA transfers and flexibility if you are delayed. I've taken it many times and it's great. You can watch a movie or listen to music.

I think both gail's and MEtrvlr's suggestions are good. You are concerned about traveling by bus in March - I guess you mean in case there's a snow storm? I can tell you that if there's a big storm, the Amtrak train to Portland will be the first to be cancelled. So, if we are going to get a storm, it doesn't matter if you're going by train or by coach... both will likely be canceled.

>>When amyb said it's better to cab from South Station to North Station than from Back Bay Station to North Station, my first thought was that, if I'm the one driving, I'd rather do the drive from Back Bay Station, even though it's a much longer distance.<<

From South Station if driving, get on Atlantic Ave to North Washington to Causeway. It'll drop you right in front of The Garden/North Station and be all along the newly famous Rose Kennedy Greenway.