Dan Roam is author of "Back of the napkin", a book which looks at visual thinking as a way to gain new perspectives on challenges and to carry an audience along with an idea along enabling you to communicate more effectively. Waiting for my copy to arrive.
Anyway, he offers a useful tip for drawing when in ppt presention mode. If you need to draw on a slide here is how to do it.
For those with older versions of ppt (like me) then you can "CTRL+P" to change your arrow to a pen. Then use "E" to erase your scribblings and finally pres "ESC" to return to normal arrow mode.
See what you reckon.