I've just started using a great piece of software called 'Infinite Painter' by this guy:

http://seanbrakefield.com/
To do sketches on my phone. Its great as it replicates real paint pretty nicely and has all the tools I would normally use on my PC. I'm using a Samsung Galaxy Note 3 at the moment, but might see if I can pick up a larger note tablet in future to do more finished paintings.