I would suggest giving your characters more movement, so they dont look so stiff while posing. On the other hand, maybe try diffrent angles to help you improve on prespective. I would also like to mention maybe using refrences to help you while your drawing.^^; I would go more into it but I tend to draw more realisticly, but you dont have to learn realism to draw other stuff, you can improve while still habing your style but it dose change alittle and in time alot will change^^;

All in all, just try something you wouldnt normally draw, to help you improve^^; sure it wont look good when you first do it, but if you practice enough you'll get it down

well, /some/ of your drawings, look like pictures of cardboard cutouts, like what pony-boy said (another word for stiff is static fyi), and this is just personal opinion, but the eyes can look a little weird sometimes.

your art is pretty good though :P

As everyone else has said, I believe you can benefit greatly from studying human anatomy. ( Learning to draw backgrounds is also another great subject to learn! )