Practice what you're good at, experiment with it, and then learn how to draw things out of your comfort zone (I'm still working on the third). Things like gesture drawing help with sketching, though. If you Google 'PixeLovely', there should be the same site I use. The practice tools are fabulous.
Link
keirajo
I want yer skillllllllllllllllllllllllllllllllllllllllllllzzzzzzz!!!!!!!