Seth Stevenson's done a great piece on how Southwest has succeeded in consistently making money even as the whole rest of the U.S. passenger aviation industry fails. This was my favorite point:
Consider, for instance, Southwest’s fleet of jets. While other airline fleets can employ 10 or more types of aircraft, Southwest uses just one, the Boeing 737. As V.P. of ground operations Chris Wahlenmaier explained to me, this results in all manner of cost-saving efficiencies: “We only need to train our mechanics on one type of airplane. We only need extra parts inventory for that one type of airplane. If we have to swap a plane out at the last minute for maintenance, the fleet is totally interchangeable—all our on-board crews and ground crews are already familiar with it. And there are no challenges in how and where we can park our planes on the ground, since they’re all the same shape and size.”
There are plenty more great insights at the link, but this is kind of mind-blowing in its elegance. One problem, I suppose, is that you might end up too much under the thumb of Boeing once you've super-committed to just flying a single model of aircraft.