Cleaning the clippings and other yard debris from the underside of the deck on your lawn mower after each use is critical for smooth, even mowing. Accumulated clippings beneath the deck can decrease the airflow needed for the mower to properly mulch, bag, or side-discharge. Built-up clippings also hold moisture and corrosive fertilizer against the underside of the deck, causing rust.
You can clean the underside of the deck using your garden hose—some mowers have a washout port that you can attach the hose to. It's easiest to do just after mowing and the clippings are still moist.
You can also remove clippings with a hard-plastic ice scraper, which reduces the risk of scratching (and eventual rusting) that a metal scraper or other tool might cause. Turn the mower on its side with the carburetor and air filter facing upward to keep oil from getting into the fuel system, and then use the scraper. This will work even if the mower deck is covered with dry, hardened clippings water might not remove.
After you've finished, spray silicone lubricant on the underside of the deck to reduce clipping build-up next time you mow and to make cleanup easier.