It's really easy when you see it explained. I couldn't find out how to do it for love nor money, just stumbled across somebody else's solution for it one day - well, I guess I could have asked, Derr!
First, you have to make sure that the face you want to work on is only one face - The ends of a cylinder, for example have to have all the edges deleted first, so it's just a 8,16,20 - whatever, sided face. 16, 3 sided ones won't work.
When that's done, delete the history on the object concerned.
Next, you select that face and hit extrude, BUT don't move any of the manipulators. With the object selected after the extrusion is 'done', go to the channel box and find an INPUT called polyExtrudeFace1, scroll down to near the bottom, and there's a channel called Offset. Just type a small number into this field and watch the results. You can also left-click on the word Offset and then drag with your middle mouse button over one of the panels to change this interactively.
Here's a pic.