Oh... using the smooth proxy mode, it can also be helpful to have only one side of your object showing the smooth under the original lowpoly. At least it gives you a better overview.

Do this by selecting your half head (before you make an instance) and choose polygons > smooth proxy. Then chose your proxy object and dublicate it as an instance (scale -1).

Then you can work with your lowpoly on the instance and immediately see the smoothed affection on the other side of the head. Might seem confusing, but it works.