Simply put the only way to keep rust at bay is to properly and regularily wash your car .
In addition the application of some suitable preventatives in the right places it will also slow down the onset of any corrosion .
When washing remove the build up of muck from the return lip of the wheel arches paying attention to the rear door seal area which is a perfect trap for salty muck, hose down the inner arches front and rear too.
Run a water filled sponge along the base of each door and tailgate .
I've mentioned this before , get some waxoyl ,thin down with white spirit and put into a lever type oil can ( preferably with a plastic tip ) .
Using a plastic edged tool carefully lift the bottom window rubbers and squirt waxoyl between them and the door frame . After cleaning out the rear wash wipe on the estate squirt the same into the spindle cavity area . Gain access to the inner rear arches via the cubby hole doors in the boot and pull back the sound deadening . Liberally squirt the waxoyl onto the valley where the inner arch meets the outer wing . This will allow the liquid to gently run down the inner arches and over time seep into the vulnerable joints.
Squirt the mixture into the leading edges of the boot / bonnet / tailgate .Wipe off all excess once finished and reapply regularily. This is best done when the ambient temp is at least 15 C
I've also removed the door panels and properly sprayed the inner door cavity's but beware I found that the waxoyl softened and caused the lower neoprene door seals to warp.
Regularily inspect the underseal especially where any jack has been used and apply schutz where neccessary.
Properly touch up any/all chipped paint immediatly.
It just takes a bit of time and common sense but it's worth it .