Because the question seems to come up with a lot of regularity (not unexpectedly I might add), and because it's just easier to make this once than to type it out each time it gets asked, here is my answer to the general question of "How do I draw better", and any one of the variations of the same.
First of all, put the cartoons, manga, and other stylized designs on the back-burner of your priorities until after you master the fundamentals of understanding line, form, shape and perspective. If you have some down-time you want to kill with some cartoons, spend a few minutes warming up first on some real life gesture drawings.
Don't copy cartoons/manga/comics. They're highly stylized designs that no matter how simple their appearance, disguise the fact that they were made by people who already have mastered realistic life drawing. One or two exceptions I can think of: The masters. If you dig up some of Michaelangelo's drawings, or for someone more moder