Well, you're still only a beginner. Posing takes a lot of tries and practice to get just right. Have you seen my first attempts at posing in a 3D program? They're just as stiff as yours! ^^
Don't bring yourself down! Everyone has to start somewhere, so never give up at improving your work!
LeftyGreenMario can probably elaborate better than I can considering she's great at talking about their proportions.