Shapen it, imagine its a sphere, not a wobbly misshapen hat.
The sides doesn't show the perspective of the back side, its like a bandana or something. It doesn't represent that the head is hidden inside the helmet.
I'll show you later something that your back view will be in perspective
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http://i.imgur.com/Wh7xp78.jpg
I'm bad at drawing, but thats the shape that will make it. Just cover the whole head and you'll be fine :)