Wal-Mart. Auto section. Preston brand spray silicon in a yellow can. Under $2 per can. Spray it down a few times before you mow, then every time you clean the deck. Works pretty good.

wouldn't wd40 work just as well?

Silicone keeps most dry debris from sticking to coated surfaces, while oil based sprays like WD-40 attract dirt and debris. I use it on clean or new mower decks, trimmer shafts, snow blower chutes with good results. It a good corrosion prohibitor, plus it does make any cleanup easier.

i'll have to try it.

Just hose off the underside each time after mowing before the grass has a chance to dry and you will significantly reduce the chances of corrosion.

my manual specifically states not to hose off the underside of the deck. maybe the grass that doesn't come off soaks the water and promotes rust.

I'm always trying out new stuff - the latest is a spray can of 'lubricant' called 'Jig-A-Loo'. The stuff is loaded with high solvents that clean, then evaporate. If then wiped down, nothing visible is left, but it's very slick - completely dry but feels like it's coated in Teflon. I've been trying to find applications for it but the first thing that came to mind was deck treatment. Haven't tried it there yet because my decks both have a pretty good build-up, but if I ever clean them I'll give it a try. Ideally you would use it on a new mower and keep it clean. I got it at Wal Mart in the auto section. I put it on some scissors; the next time I used them to cut strapping tape while making a big shipping carton, the adhesive did not stick to the blades. If anything gummy gets on scissors or a knife, this is the stuff. The solvents are pretty active, though, and can affect certain paints and plastic materials (but not the baked-on paint of a mower deck). I can't tell what's in it that makes it so slippery but it does state 'no oil, grease, wax or detergents'. For those who play with guns, it is absolutely 'the berries' for cleaning/lubing the inside of magazines with no effect on primers and no dust collecting. I can see it for cleaning and slicking up anything on a mower that you don't want to collect grit; cables, mechanisms, adjusters, plastic wheels, etc.

There's a product made for lawnmower decks. It's Mo-Deck. Click the link.

Here is a link that might be useful: Mo-Deck

Fluid Film By Eureka--non-toxic you can get it in most lawn and garden shops great for all kinds of equipment

Try "Mower Guard" by Slip Plate--Superior Solutions
http://www.slipplate.com