I often use napkin to draw sketches but I am an awfull drawer. That's why I turned myself to this summary of Napkin Sketches 101 written by Don Moyer in 360
1. Realize ugly is beautiful.
Crude, ugly and wobbly are okay. If the idea captured is valid, youll have time later to make it beautiful.
2. Master the basics.
If you can draw a half-dozen simple shapes youre ready to take on almost any topic.
3. Use labels.
Include lots of labels and notes so things will make sense to you when reviewed at a later date.
4. Keep it simple.
In general, leave out any detail that will not be missed.
5. Be consistent.
Avoid variations that dont mean anything.
6. Break some rules.
Dont worry about keeping things in the right scale.
7. Let your arrows speak.
Pointing arrows are the verbs in a napkin sketch.
8. Use the right tools.
Use whatever surface is available paper, whiteboard, small note-paper etc.
9. Dont keep the napkin on your lap.
Dont hide your sketches in a file folder. Make them visible and share them with teammates.