You need to mow more often (only cut off 1/3 of the height if mulching) and mow when your lawn is dry (late afternoon or evening). That should cure most of your issues unless your lawn is more weeds than grass. In that case you have more issues than just a rusty mower.

My son works for a rust-proofing company, and he says folks have them spray the undersides of their mower decks with the rustproofing as used on pickup beds. Of course, there is no guarantee, and you must remove the deck to have it done.

Do you know what the actual product is that they spray onto the mower decks? I'm thinking about trying POR-15 but I don't know how well it will hold up on a lawnmower.

What i did and so far it's working. On an older model Troy bilt rider i took the deck off, removed the three blades and spindels. Pressure washed the deck let it thourghly dry and had a friend spray it with some leftover liquid bed liner. This was last fall and so far it is holding up pretty well. I used it on leaves last fall, and have mowed twice so far this spring. After each use i use a blower to clean underneath. Not sure how long it will last.

I've had great luck with Loctite Rust Neutralizer on a variety of products (haven't tried it under a mower deck but don't know why it wouldn't work). http://www.loctiteproducts.com/products/detail.asp%3fcatid=13&subid=47&plid=168

Here is a link that might be useful: Loctite Rust Neutralizer

Bluelawn: This topic has been discussed at length previously. POR-15 is a very good product a little pricy
but excellent. If you already have some it will work great for the intention you have in mind. If not there are other treatments as has been advised bu the other very knowledgeble forum members . Check out the Prices and go for it .