We use Pledge.
Seriously. It's a brush-on clear sealant usually used on hard wood floors. It has to be Pledge Floor Finish with Future Shine. Anything else won't work. We've only used it on vinyl and ABS plastics so far (mostly playline), but it works really well. It's very very shiny. We don't use pastels either, just acrylics, and the shininess works really well for lips and eyes.
I have Monster High girls that we used it on years ago now and they have held up really well. No yellowing, no stickiness. Plus you get a huge bottle for really cheap. We still haven't used half of our bottle and we bought it well over 3 years ago now. We have some little plastic cups with lids on them that we pour it into to paint with it.
You will need Windex to clean it out of your brushes though. It comes off fairly easily from dolls with a Magic Eraser and 91% alcohol. Also, it smells decently (We also had WhiteDove's problem with super stinky sealants).
I got the recommendation years ago from someone in the dollieh community, though I don't remember who. The link above takes you to Wal-Mart's page for it. That is the newest design of the bottle.