De-greasing the model (or whatever)

That should do it on polystyrene! I just put a few drops of  dishwashing liquid in and make sure its mixed in the (warm not hot) water well. Then rinse well when finished. No need to go too crazy.

Its the first thing I do with all models.

Frankly when I was starting out 25 years ago I never washed anything and those models are still ok now.

It can't hurt and it takes little time.

I think people wash photo etched parts with alchohol if I'm not mistaken. I always just quickly wipe them with mineral spirits.

No need for anything besides good old dish soap.  I use Joy since it does not have the additives for keep your hand soft and sweet.  If you don’t given the arts a good wash, the mold release residue… pick up an ICM kit and look at some of the guck left on the sprues… may not allow the paint, or glue, to adhere properly.  This will become very apparent when you pull off some masking tape and it peels the first color you put down right off as well.  I also have alcohol wipes, like the docs use just before they stab you with a needle, to wipe the model down just before painting.

If you paint with enamels, there is little (usually) consequence to not washing, as most organic solvents will dissolve and neutralize skin oil. However, acrylics are another story.

Before priming, I use either alcohol (isopropyl or ethyl) or a surfactant cleaner, or Simple Green (which needs to be rinsed with water).

Between coats, just soap and water. I use either Woolite or Dr. Bronner's, as they do not leave residue. Any detergent that does not have "skin conditioners/softeners" will also work, provided that you rinse it off completely.

But even with enamels, cleanliness is next to…just do it!

In addition to the methods the other posters have mentioned, I've always had good luck with Floquil Plastic Prep. I use a lot of acrylics, and it's handy for giving the model a quick wipedown before I paint.

I also keep a cheap bottle of rubbing alcohol on the bench. A little on a paper towel keeps oil, etc, off my hands, and more importantly, off the model!

Matt @ FSM

I've done all these plus, on occasion, a quick wipe with Windex.

Always Iso alcohol here.

Just apply it with a cotton swab to remove finger prints etc.

After this point I never handle my models without wearing latex gloves.

I hate cleaning at the best of times, so no point in doing it twice!