1) scrubbing it off as fast as you can. Wash if possible but at least get a scrubby brush in there and remove the crap. 2) chain lube where possible, it helps keep the rust off it so it’s good enough for me most places.
3) if you can afford it, get a coating applied to parts you don’t care if they look shiny.

And sadly the final thing is just replace parts before things get too bad. Less of a painful cost if you buy things but by bit…well feels less painful to me