If you imagine a box around the drawing from it's furthest edges you should be able to also imagine a centerline down the middle. That centerline helps you see if the drawing is too straight up and down. It also helps to keep the work asymmetrical and balanced.
My sketches below show the box that I'm talking about. I didn't actually draw the box but I was thinking "no straight up and down, no straight up and down!"
Hopefully this works for you because it really helps me to get that LEAN in that Walt talked about.
Forgive the quality of the Iphone pics.