Quick tip: Always keep a pen in your pocket. I was having lunch somewhere in Pasadena, California where I thought of some surfacing that might work for a Mazda concept I wanted to design. I literally drew this on a napkin, scanned it into Adobe Photoshop when I returned to the studio, then did a quick render sketch. I always had a pen in my pocket. I do think that if I didn't I may have lost this idea. This entire sketch, minus the grey values, took all of about 3 minutes.
Napkin Sketch to Digital Render
Always have a pen in your pocket (or purse). A 3 min napkin sketch for a Mazda concept design I was working on.