Just like your thoughts and ideas, sketches are in a constant state of flux, evolving and morphing as you reach a potential solution. Don’t think that you have to be able to draw in order to sketch, although having some experience with it does help.

The Messy Art Of UX Sketching
Smashing Magazine

Why It's Important to Sketch Before You Wireframe
UX Movement

Skeching Articles (various)
UX Magazine

Tools for Sketching User Experiences
UX Booth

The Theory and Practice of Sketching within the UX Industry
Paul Olyslager

Sketches and Wireframes and Prototypes! Oh My! Creating Your Own Magical Wizard Experience
UX Matters

5 Tips from 5 Designers to Improve Your UX Sketches
UX Pin