What can I do to keep my rear derailler in good condition when commuting in the snow? In the winter, it seems that whatever I do, my rear derailleur stops functioning after few weeks of commuting in light snow. Ice always gets in there somehow, and the chain refuses to move the wheel, skipping.

The bike always stays outside (covered but unheated at home, outside at work) all the time, and I tried to bring it in for a melt every day, and every week. This seems to make the problem worse, so that even if there is no snow and the freewheel looks fine, the freewheel skips. Maybe rust has gotten in there, now.

What can I do to prevent the freewheel to skip after few weeks of usage in snowy conditions? 

I'm not interested in internal gear hubs for now, as they are much more expensive.
There's not much you can do aside from cleaning your drivetrain often and thoroughly. In the winter, ice is far less of a mechanical issue than is rock salt. 

You may indeed have some corrosion in the entire drivetrain by this point, not just the deraileur, or possibly just dirt if you're lucky. I'd do one major cleaning at this point, and see if that takes care of the problem. 

I'd remove the chain entirely and let it soak in addition to cleaning the deraileur and cogs. Failing that, you can get a chain-cleaning device and use that. It's not as thorough, but still better than nothing by far. 

After that, I'd clean my drivetrain once every week or two, depending on conditions. Also keep in mind that you may need to lube more often. None of these are really a big deal, once you set up a space (an old towel on the floor will do it). 

If none of this helps, it's time to look into possible mechanical causes. Riding on ice and snow, I often find pebbles and grit in my drivetrain, more than in any other season. Possibly you have a bent tooth or other mechanical issue? You can give the drivetrain a good inspection while you clean it, looking for any problems.