That washing bit during a long randonnee is something I forget to do sometimes, and I can pay the price.

I don't use chamois creams, although I might apply some tea tree lotion to chafed areas or hot spots. But generally, I find that the condition of my butt improves in the same proportion as my general fitness conditioning for long distance riding -- that is, if I ride more often, the fitter I get and the more comfortable I am.
Calluses on the butt help.

If you have the luxury of bagdrops or loops that return you to a central point where bags are able to be kept, a clean, dry pair of shorts every 300km or so is a good idea.

By the way, congratulations on the 600. A great effort. Were you the only rider? And who organised it (as in individual or association)?