Theres another way than using sub-d's. The polygon proxy mode is archived by selecting your poly object and goto polygons > smooth proxy. In the layer menu you can choose different ways of viewing. By standard you will see the smoothed version under your lowpoly model.

This is much different from sub-d's, because you do not create an sub-object. You just see all the time how your modelling is affecting your smoothed object. Pretty cool.