The first thing is a bit involved but make sure it is properly rust-proofed using Waxoil or similar. If not already done you are now in the wrong time of year to do this as the underside needs to be clean & dry, also these products flow better when warm. A a job for next year maybe?
The most obvious thing is simply to wash the underside with a normal garden hose every week, this keeps the muck at bay and prevents a build-up which leads to rot. Most people only wash the bodywork or think you need a pressure washer to clean the underside and both are wrong. Washing the bodywork is simple enough using a wax-shampoo (washing-up liquid contains salt) and spending 5-minutes cleaning under the underside each week with a normal hose plays dividends as the salt & road-muck are washed off almost as soon as they get there giving it no time to build-up or do much damage. Make sure you get into all the nooks & crannys and the vehicle will come out of the winter looking as good as it went in.
A benefit is that whenever you or anyone else has to do any work under the vehicle it is completely clean and you don't get a facefull of dirt everytime you touch something.
