I'm doing some sketching at present to improve my skills.
I'm reading Drawn to life by Walt Stachfield. I strongly recommend this book to any comic artist looking to increase her abilities. This book and it's second volume are like a college course given in frank, easy to understand language by a man who you could have known for years.
There is a lot of emphasis on gesture drawing, capturing the emotion of the lines and how they help make a static image appear as though it might move on it's own.
I love these books because I love learning art but I do horribly in a classroom setting. On top of that, with a combined cost of about 36 dollars, the wealth of knowledge offered at this price is a steal.