The first thing I am noticing in your drawings is that you are cutting right to the chase and going for final lines rather than build up to it. I don't see much along the lines of construction and such. They start to feel a little flat. I'll also suggest trying to sketch with a little more looseness in there.
There's no one way to go about drawing, but I find it extremely helpful to lightly sketch out a foundation, and rough out the form based off that. This way, I can work and rework a drawing before I get to the final lines. Getting into the habit of keeping construction in mind helps you build up a drawing.
Basic examples/guides for construction:
I use a light-colored pencil such as a col-erase to sketch and construct with, and I can then ink and erase the rough sketch. The more you draw, the better hang of it you'll get. The color pencil sometimes doesn't completely erase away but if you're digitally coloring an inked piece, there are ways around that.
I'd suggest keeping a sketchbook for drawing, and when you draw-- start thinking about how to build it from simple shapes. Start thinking about the line of action. Don't worry about the final lines, just work on building it up. Shapes, line of action, and looseness.
Also take a look at Glen Keane's work--
His drawings have a lot of fluid motion to them. There are some very lively sketches in there that are very rough and loose. :D
I hope this helps!