Internship is probably the worst year, so choose a good internship--not too cush, but not brutal. Mine was too rigorous. Pay attention to Dr. Doze. My wife didn't really care for where I went to med school and did my internship. It was an ugly city far away from her family with no mountains and no ocean. She made excellent friends while she was in grad school and that made things more tolerable. We're now in a nicer location for residency, and she's much happier. Keep in mind what your spouse likes--geography (ocean, mountain, plains, big city, smaller city), opportunities (job, school, etc), relationships (family, friends, etc). You need to be in a place where your spouse can have some support, either through work, church, other residents' spouses, residents' spouses' group, etc. Acknowledge the sacrifice your spouse makes.
I'm finding that my wife is expressing more anger at my residency and work hours since we had a child, especially when my schedule is particularly bad (hearts until 3:00 AM, liver transplants in the middle of the night). I'm still working far less than when I was an intern, and I've got most weekends off, but she's getting more upset than she did previously. I don't know whether that's because of our baby or because she's getting sick and tired of putting up with the crap of residency.
Talk to some married residents at your home school's program and when you interview. See what they have to say.