How exactly are you approaching it?
Are you going in with details first or are you starting with simple shapes? Because you should be doing the latter.

If you're going to look at other people's art try to find sketches done by them that involve the basic shapes, or that are very gestural. (look up gesture drawings if you don't know what I mean by that.)

Practice spotting those shapes in a photo/drawing, and the proportions in relation to those shapes.
Grab one of those pictures, open it up in your painting program, and draw those shapes and lines on a layer over it. Pic related is an example of me doing that.