I decided a long time ago to raise the mower with the dedicated chain hoist, roll out a strip of carpet, and clean the deck with a 4" putty knife EVERY time I use it. If I don't, the grass/dirt turns to concrete and is almost impossible to get it off. It's just part of my routine before heading to the recliner.
I tried spraying the underside with a light coating of oil each time I cleaned it, but that didn't help.