Mine get's garaged. In the past however, when I have no choice, I've used a product called LPS 3
http://www.lpslabs.com/product_pg/co...n_pg/LPS3.html
I would cover all the parts underneath that I think would start to oxidize quick and even strategically under the hood. Then, in the summer, I would power wash it off. and wipe the visible areas down. I also had a lift at my disposal so that helped.
Love this stuff.