Hi.
You can get better results out of illustrator by doing a couple of simple things.
In the box that has your stroke size, there are also 6 boxes to the right. If you click the top middle one, it will give your lines a nice round end, instead of the harsh 'chopped off' flat end you have at the moment.
The next is a great one for giving things a more 'organic' feel.
In brush libraries (WINDOW), open ARTISTIC INK. select the outline of your picture, and click on one of the simpler brushes (like the straight with 1 rounded tip, or pencil texture). DON'T PANIC! Your outline has now probably gone fat & crazy. Simply decrease your stroke with until you're happy with it (for example, in your preset stroke with box it only goes down to 0.25, but you can go as low as you like by entering it by hand eg. 0.05....0.10 etc)
This will take a bit of practice, but you'll get the hang of it in no time & have much nicer results. the brushes add thickness/thinness/texture in different areas, and you can alter where this occurs by snipping the path here & there with the scissors tool to see what looks good.
Once you have grasped it, try using it with your interior strokes, but always have your outline stroke slightly heavier.
Have fun, and don't use the really complex brushes, they make things too complicated.
Let me know how you get on.