All I can recommend is what works for me – which is to practice regularly. If you aren’t satisfied with what you’re making, take the chance to deconstruct why, and try to improve those areas. If you do that daily and make small changes in your technique, eventually you’ll develop a style you’re more happy with. The key is just to do it often, and iterate quickly. Check out this talk I did at PechaKucha for more of my thoughts on daily sketching.