It's been suggested to me time and time again (and I've been "dabbling" in it lol) that yoga does wonders for the massage therapist. From what I gather, it especially stengthens your core, so that would help your back a lot.
I haven't tried it, but I think Tai Chi would be helpful too. It seems that some of the postions MTs get into are very Tai Chish lol
I suffer from a bad back and am overweight, so I am doing ab work, because it is weak, and if it were stronger, I think my back wouldn't hurt so much (My back doesn't hurt anymore while massaging since I found proper body mechanics).
Also, I was suprised what a workout giving a massage can be. Maybe not for everyone, but if you are out of shape, it can be. And one shouldn't be huffing and puffing during a session! I think doing some cardio would build up endurance, so when you find yourself constantly moving for an hour during a session, you aren't worn out. Hope that helps
(note: IMO, ideally one should be flexible, have good balance, a strong core, and strong legs and...know how to relax and breathe properly.)