Suggestions To Get a Shallower Depth of Field

The photography club I belong to is having a critique meeting next month with the subject being self portraits. I had the idea to photograph myself asleep at my desk with my boss standing behind me looking on disapprovingly.

This evening I took some test shots to see if the idea will work. There wasn't anyone around to portray my boss, so I had to stand in.

I would like the foreground subject, me sleeping, to be in focus. I would like the background subject, the boss figure, to be a little more out of focus, but I'm not quite sure how to accomplish it.

I have a D50 and the Nikon 18-200 lens. I shot in aperture priority with the lens as wide open as it would go. If I understand correctly, longer focal lengths contribute to a shallower depth of field. I've got the camera positioned back about as far as I can, so I can't really move the camera back and zoom more.

Can anyone suggest something I can do to get a little shallower depth of field? I'll also gladly accept any other suggestions to improve the photo.

Thanks,
Shawn