Ooh, this is cool :3 I'd give you the opposite advice though actually! Relax and try your hand at quick, sketchy lines, it's easier to get a "flow" and get a smoother result. Don't worry about it being perfect or keeping it to a single line, go over it where it's not quite right, etc. You'll end up with a really rough sketchy thing with lots of lines crossing over each other. Then start a new layer over the top and follow the shape you've sketched out. Please forgive the crappy example(done with my trackpad in 30 secs, woot), but a bit like:
rubbish exampleYou might not find it helps at all, but maybe worth a try? It'll help you get the basic shape down as well so you don't have to keep checking back with your reference picture and it should help you relax and let your lines flow a bit more as well. ^^
It's great as it is, btw, just something I find helps with getting smoother lines, feel free to ignore me completely
![Razz :P](./images/smilies/icon_razz.gif)