The line art is superb. Brilliant stuff.
Not too keen on the colours yet, but it is still very early in the process. When it comes to composition I find that the key to it is to keep any shapes that the characters have started to form are completed. What that means is if you start to construct characters in a shape (say a trangle) but only fill two sides af it, the composition will not work, it will look strangely hollow on the side that is missing.
For example, look at this: http://community.imaginefx.com/fxpose/domtopias_portfolio/default.aspx (It's called Spiral) It's a painting I did a few months back and although there are loads of things wrong with it, it taught me a fair bit about composition. At first I noticed that the character was only forming two sides af a shape (a triangle in this case) with her legs as a base and her back as one side. I was really struggling with what to put in the gap on the left (I had tried every permutation of arm position you could think of! Plus, all sorts of furniture and background objects!), untill I drew in a sword (a linear object) and angled her arm to complete the shape. It instantly made the whole image come together and the narrative flowed naturally. Hopefully you can see what I mean?
Alternatively, if you are painting a dramatic or comicbook scene you could take a look at the workshop in issue 82 of IFX about character composition on pages 78-82. It explains it better than I just did!
Hope that made some sort of sense!