It doesnt look like you are using a pen toll for the lines,
You should use a pen tool so you get smooth lines
And you should also very the width of your lines, eg 4 pixels outline, 3 pixels window and headlights outline, 2 and 1 pixels for body work.
There are a lot of other things I can say, but I have already said them in another post, (check Auto chops section, Page 3 or 2, Thread called "am I doing this right")
Here are some really really helpful tutorials for tooning
Using a Pen tool: http://www.deviantart.com/view/19028193/
Full Tooning tutorial: http://www.deviantart.com/view/29393935/ (VERY GOOD)
MartinPliksh