I turn off the fuel line and run mine dry at idle, fill the gas tanks and add a stablilizer. Since they're stored in an unheated shed I pull the batteries and store them in my garage, which isn't heated but consistently stays above freezing.
I'm going to do some work to both of my bigger mowers over the winter this year. The starter on the Tracer is on it's last leg, the rear deck struts on the Lazer need replacement as do the fuel valve and ignition switch...been running on one tank lately. Both could use a valve adjustment I think and a hydro service/oil change. The tracer gets a lot of debris behind the engine shrouds so I'll pull those and blow things out good. I'll probably do the Lazer too but it's much less of an issue on that machine.
I usually work as long as I can so I won't do anything to winterize them until there's snow on the ground.