Thanks! xD
Well, here's how I do it:
What I do is I draw whatever it is on paper. (Of course.
) Let's say... A Turtwig.
Now, I use an Ultra-Fine Point Sharpie (black) to go over the drawing. This way it'll stand out more when I scan it in.
When I scan it in, I make sure it's a fairly big size (the default scan size should be fine, but I'm not sure what other scanners use).
Now, here's the part you're having trouble with. You need to make the lines darker, without having to go back over them. This process is made a lot easier if you outline the drawing in the thin black sharpie. So, if you've done that... Then good. If not, this may still work.
There's a tool called "Threshold". What it does is it makes everything on the image either pure white, or pure black, with solid and non-blurred pixels. This allows for extremely easy Outlines to be made.
I use Jasc Paint Shop Pro. It's a free program, but you'll just have to look for it on the web, as I forgot where I originally got it. I use Version 9, so if you want to get it, look for that.
In Jasc Paint Shop Pro 9, go to Adjust => Brightness and Contrast => Threshold. Then increase or decrease the number until you get the desired thickness. I use 157, but it's up to you to decide what's best.
EDIT: I may end up making a guide for this soon, so if this isn't clear enough, I'll be sure to explain it better there.