I've not got a M5 but a 530d but i guess the same applies (except I'm
If you can avoid taking it out altogether then thats the best but not sure where you are located that may not be an option!
I'm located in london and when the roads are gritted I make a case of getting the car hosed down, concentrating on the underside. I'm not sure much more can be done to be honest. hosing down is not going to get everywhere but its better than nothing.
BMW goes a long way to prevent corrosion, they test up in places like Sweden which i'm sure has plenty of Salt on the roads.
just keep cleaning it .
I'm sure others may be able to suggest other tactics.