I've cleaned several now with a *solvent* soak, in one of those robust plastic Rubbermaid food storage tubs with a tight-fitting top. Let soak a day, spin awhile and remove, filtering the *solvent* through a coffee filter, then repeat, until spinning no longer gets you gunk or particles out of the thing. Let air out a few days (no heat, now) and stink the house up good. Then spin in drops of oil until it won't take any more, let drain on rag/towel and spin some more. I try to keep the cogs themselves relatively free of oil in use (use ProLink Gold on the chain), and clean the freewheel superficially about once a week when riding regularly, or whenever it's been windy/wet.
*solvent* I've been using is kerosene, which others will pooh-pooh, but I, like General Melchett, will pooh-pooh the pooh-pooh, until I find something I think works better. I've had no luck with using Simple *color*, just because it doesn't seem to cut the grease well enough, and I am morally opposed to putting water and steel together when I can avoid it! Someone whose nick is etherhuffer has no call to complain about solvent.