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A good cartoonist always has learned to draw realistically first. To give a cartoon body a sense of realism you need to get how real bodies work.
Basically you got to know the rules before you can break them.
I know it's basic and boring advice.
But doing art studied will help you can an understanding on how to construct any sort of image. Studying from life will give you plenty of practice and help you break down more complicated drawing tasks into shapes or whatever your preferred method is if you do it enough. You can go to a used book store and go to the art section and pick up anatomy books and life drawing books pretty easily.
And this works if you want to approach a certain style too.
Look at your art inspirations and study their work. Learn what you like and what you don't, put your own spin on it.
It boils down to practice and challenge yourself in some different way with each new practice session. It can be really hard for yourself to accurately judge how much you've improved since you see the work on a regular basis. You've probably improved a lot more than you think. Posting an album of your chronological work may help people give honest opinions.
Don't give up!
Now it is time for a cookie!