The best thing to do is use pen and paper. But don’t use it to do wire-framing or layout’s, instead use it for simple drawings not paying too much on the organization, but the creative freedom that the pen and paper can give you.
Also make sure to use a pen and not a pencil, as it will force you to not care too much about being perfect with the drawing and let you get your ideas out better.
P.S. Photoshop for the Win!!!