A mower lubricant or coating that you can spray on the underside of the deck will help to keep grass and dirt from building up and help the mower run better. There are a lot of different products you can buy, but a good one is EcoTech Mower Coat.
You can spray EcoTech on the underside of the deck, on the blade, on any exposed pulleys or shafts, and on the wheels, too. It definitely slows down the accumulation of grass and dirt. On my lawn, I used to have to stop mowing at least two or three times each time I mowed to clean off the underside. Since I put a coat of EcoTech on it, I haven't had to stop once. (I usually clean it after I mow, but the accumulation is never that much.) It's been almost a month since I sprayed it on and it's still working.
EcoTech also works great as a lubricant for the crankshaft. On my mower it has improved engine starting since I began using it. A 10-ounce can of EcoTech costs $5.62 on Amazon.com.