Finding the right tailor can be quite a task, but once you do you're set for life. Le Couturier is my choice for alterations in the Boston area.
I was first introduced to Dick Robasson (tailor to Ralph Lauren, Gucci, & Loro Piana among others) when he was located near The Closet on Newbury Street. He has moved his business to Central Square, but let me tell you, it is worth the trek.
There is a reason he was voted Best of Boston last year by Boston Magazine. I recently went in for a special project, the remaking a vintage dress, and although I will have to wait a week, I have no doubt it is in the right hands, and will fit me like a glove when finished.