Same as a few of the above folks, I have my equipment sorted into different bags: Canon FD, Canon EOS, and Nikon. And even then, I can't fit all of each outfit into one bag, so I have to make some decisions as to what I want to carry before I set out, or pack extra bags of gear. And none of these include a couple of long telephotos or my Metz 60CT1, which gets its own bag.

With the Canon FD, usually I'll have two bodies, a few primes and a couple of zooms. EOS, I'll carry a 35mm body and a DSLR. I have several EF zooms. No primes yet. Nikon, I have a good assortment of both primes and zooms to bring. Depending on the situation, I might also bring one or both long telephotos I own -- a 500/5.6 or 650/6.8. They're T-mount, so I can use them with any of my systems.