For group classes (great for working with distractions), the New England Dog Training Club holds classes on thursday nights at the Cambridge armory (near the Fresh Pond rotary). http://nedtc.org/

For a one-on-one approach, the best resource for finding reputable trainers is at Boston Trainers Network: http://bostontrainersnetwork.org/find-a/

Both of the above employ only force-free methods, and the quality and level of knowledge is exceptional.