there is only a few things i would tweak about your head,
'the shading around the pattern on the helmet seems to be just black smudges, try using a darker shade of the base color to shade things, using black can sometimes work but usually it looks very bad.
here is a video that helped me with my use of color when shading: (you may want to skip to around 1:00)
http://www.youtube.com/watch?v=k21Dn75p4cU
there seems to be a lot of lines that are very unclean, if a line isn't very smooth it often lowers the quality of the artwork. you may want to try creating another layer and using your tablet,draw over the lines again trying to cover a section of line using one stroke, if you need to use more than one stroke that's is perfectly fine but try and get them to carry on from the previous stroke without anyone being able to notice that there are two very different lines there. (i know this sounds strange but once you are doing your line work like this you will be able to tell a difference in your work)
it is my personal preferance to try and do it in as few line sas possible as i struggle to draw the lines carrying in from one and other smoothly, but there is also another technique known as fethering, this is basicaly the same proccess but you use loads of lines to make one smoother line, i dont know much about it as i dont use it but i went ahead and grabbed a video for you so you can check it out, (from about 4:00)
http://www.youtube.com/watch?v=wpLKcJiekFc