I've spent far too long staring at flight tracking sites, and occasionally I think I'll notice some pattern in what types of aircraft a given airline uses for certain scenarios/routes.
For example, Delta seems to operate a lot of MD-88's on short-haul routes that other airlines would likely use ERJs or CRJs on. They also seem to use 757s on quite short hops (ATL to southeastern cities ~1hr away).
Another example would be why British Airways operates 747s to some cities, and 777s to others, and exactly how they decide where to use which aircraft.
Is there a good reference for figuring out the reasoning behind why airlines use a given aircraft for a given route? Or are there too many different factors and too much corporate secrecy to really know for sure?