I’ve received a bunch of notes asking if I actually draw on napkins, and if so, what kind.
Yes, I do draw on napkins. They’re not the best drawing
surface, but when I want to show someone how quickly a sketch can be
created, I like napkins. And since everyone knows that napkins are
intended to be thrown away (and recycled!!), many people are less
intimidated drawing on them than expensive paper.
I use "Vanity Fair Everyday" napkins.
They are available in all major grocery stores. I like them because
they are tough enough to make a good writing surface, have a flat
smooth finish, and have compressed enough paper that they limit ink
bleed. (Avoid paper towels: they will suck all the ink out of a pen in
a couple minutes.)
As for pens, any ballpoint works, since they minimize ink bleed. That
said, the best napkin-drawing pens that money can buy are Pilot
"Permanent Ink Markers". 
Unfortunately, the very best are only available in Japan or stationery
stores that stock directly imported Japanese drawing materials. Here in
San Francisco we are lucky enough to have a major Japanese shopping
center where the Kinokuniya Bookstore chain has an amazing stationery
store.
Photo from bettybl.
All this aside, the point is to find a surface that is not
intimidating. That may sound odd (how can a paper surface be
intimidating, right?) but it really does make a difference. If you want
people who might not be comfortable drawing, you have to given them
tools that feel spontaneous.
A pencil and a notebook work too.
