As said above, never, ever, ever, EVER trace. It's fine if you're copying straight from reference, because even that helps you with things like proportion.
Also, this may seem like it's completely unrelated, but practice drawing actual real life people/anatomy. Once you get a better grip on how the actual human body looks/is formed, it'll carry over when you try to do more stylistic things like Dragon Ball. Your timeline definitely shows improvement, but even the latest picture has "basic" mistakes when it comes to anatomy/perspective/proportion.
In fact, now that I say that...those are three of the biggest things that will help you out if you take more time to practice them.
Anatomy - Basically just the human body. What curves are where, where each muscle is located, how certain body parts connect to one another, etc.
Perspective - Drawing things from the correct "Point of View". Drawing things/people in such a way that the viewer gets a believable representation of the subjects height, weight, and depth in relation to the environment.
Proportion - Making sure each part of the drawing is the correct size in relation to the rest of the drawing (For instance, your typical human hand, from the bottom of the palm to the top of the middle finger, is around the same height as your face from chin to hairline).
One thing to help you improve in these areas is to do something called a "gesture drawing". This is basically when you have a model/piece of reference, and you take a short time to draw it. However, you do so by starting off generally (Sketching out the basic pose, lines of action, horizon line, etc) and slowly making your way towards more specific details. I would recommend giving yourself a time limit. Start off with a good amount of time (Maybe 5 minutes) and then slowly work your down to around 30-10 seconds. Your goal isn't to "finish" the drawing, it's just to get as much of the general idea down as possible.
Here's a good site for doing online gesture drawings:
http://artists.pixelovely.com/practice- ... e-drawing/
You can choose what type of models you want, and set how much time you get for each drawing.
Here are some examples:
http://puu.sh/aUAAi/5c71e8d9c8.jpg
http://puu.sh/aUABg/c84e13c982.jpg
http://puu.sh/aUABD/eafa587620.jpg
http://puu.sh/aUACb/4a0ae94733.jpg
I'm guessing these artists' got less than a minute for these sketches, but they didn't try to "finish". They just tried to capture the general idea/posing. Work on your posing/anatomy and you'll see a great improvement in other areas.
Like you, I spent the entirety of my middle/high-school career pretty much only drawing DB fanart and practicing based on that. I made improvement over that time for sure, but once I got into college and started taking classes on this stuff, my improvement skyrocketed! That's about all I have to say right now, hahaha. Sorry if you've heard any of this before, and if any of it seems confusing just ask and I'll try to clarify.
