I try not to have too much redundancy within formats, so that each camera has a reasonably well-defined niche. There are exceptions (I've got two working TLRs as well as a 6x6 folder, for instance), but settling on a format and/or shooting style narrows down my space of choices quite a lot.
It also helps to have a sense of the cameras as individuals. I can't necessarily tell you *why* some subjects are just an obvious fit for a particular camera, but I can fairly often look at something and say "oh, the Voigtlaender would like that". Presumably this effect is mostly psychological---the truth is, the Voigtlaender doesn't give a damn what I point it at---but it works anyway.