Each time I mow, I finish by hosing down the mower, and directing a stream of water from the garden hose just forward of the right-rear wheel for a couple of minutes with the mower running. The spinning blade picks up the water and uses it to scour the underside of the deck., leaving it amazingly clean. Years ago, I bought a mower (IIRC, a Craftsman) that came with a recommendation for cleaning it this way, and I've had a couple of mowers that came with hose connections on the deck to make this kind of cleaning even easier.
I had to buy a new mower a couple of months ago, and was told that the new machine I chose was the same Toro model that I was replacing (after 10 years of use). I asked the salesman why the new machine didn't have the hose connection that the old machine had, and he mumbled something about improved air flow making regular cleaning unnecessary - a comment that I categorized as typical salesman BS. The real reason is that Toro saved a buck by omitting the hose fitting.
Then, at the end of each mowing season, I replace the oil, air filter and spark plug and drain the gas. I also remove and sharpen the blade, and then spray a coat of WD40 on the underside of the deck. The WD40 means that I will start the next season with a clean mower deck, which means that it picks up less grunge each time I use it.