honestly - it looks like you are wasting quite a bit in the details - i'd get the whole shape down before dealing with the intricate parts.

you need to think about what needs to be modeled and what you can get away with using textures. for example the teeth - you don't need to model those - you could get away with textures and a bump map. same with the helmet - if you want that stuff to be modeled you should at least wait till the end when you have everything else mapped out - but again, you could probably get away with a texture map and use that geometry more effectively elsewhere.