I know others have described this pretty well already, but I'll add my own explanation as well.

I keep the strap pretty long, about four feet total. When I'm walking, I usually put the strap over my left shoulder and the bag rests against my left hip, where I hold it in place with my left hand. If I'm in a hurry, running for the train or whatever, I'll hold the bag pretty tight. When I get on the bike, the strap goes over my head onto my right shoulder and I shift the bag backward so it's just below the small of my back. It rests against my belt, near the base of my spine. On most of my bikes it is pretty stable there.

How well this works depends, I think, on some specific measurements, including the relative height of seat and handlebar. On my commuter bikes the handlebar is three or four inches lower than the seat. Occasionally I'll ride a different bike, with a lower handlebar, and then the messenger bag isn't as stable, having a tendency to fall off and swing to my left side, which is annoying. Some people prevent this by using a second strap --some messenger bags have a belt for this purpose-- but I don't.

Please don't misunderstand: I am not recommending a shoulder bag, or any thing of the kind. My advice is: figure it out for yourself. Threads like this are excellent for showing the wide range of needs, and the wide range of solutions available. There is an ideal solution; but no one here can tell you what will work for you. We can only tell you what works for us.

In my case, the thing that makes the messenger bag work is its flexibility. It's on my left shoulder when I leave my office; when I get to the CitiBike, it moves to the other shoulder; when I get to the station, it goes back to my left shoulder. When I sit down on the train, the bag goes on the seat next to me, or on my lap if the train gets crowded. When I get off the train, it goes back on my left shoulder while I walk to my bike, then my right shoulder when I get on the bike for my five mile ride home.