How do you get such smooth lines in your work?

Hi! I wasn't quite sure if I should post this here or in DigitalPainting, but anyways... How do you guys get such smooth lines? I can't use Photoshop when I draw because my tablet lags with it 9 times out of 10, so I resort to SAI. My problem is that my lines just aren't smooth or crisp or straight, and it's really getting on my nerves. I take the time to go over them and try to get them as perfect as I can, but it's such a tedious task.

http://i49.tinypic.com/6stc0j.png Here's an example of what I mean. (It's my most recent :>) See how the lines just aren't full and kinda squiffy?

I'd really like some advice! I can do smooth strokes easily with paper and pencil, but for some reason with a tablet, it's just impossible for me. :(