Napkins are the best

Napkins are great because they are cheap and readily available resource, easy to obtain easy to deposit.  They are especially useful during chilly weather when you nose just can't stop leaking. Since they are so easy to get rid of, you don't get attached to your sketches like you do when you draw on your nice expensive acid-free 90 lb sketchbook paper. Some sketchbooks are heavy. It is actually not good to carry it all the time with extra weight added to your backbones.

Many great ideas spawn from napkins. Have you ever wanted to make something great so you sat down at your table with a pencil and a paper, but nothing came. It doesn't work. You tried too hard. Great ideas are hidden in the subconscious. If you are too conscious you can't get access to them. But when you are drawing on napkins, you enter into this mind set: "I'm going throw this away later. This is just for fun." Crazy stuff happens on that little piece of trash.

p.s. they don't work so well in the rain.