It depends on what tool you are using. With stylus or a brush you can get thick and thin very easily; markers, pens, etc., tend to be more consistent and less varied.
A quick thing you can do is use a heavier line for the foreground and lighter for midground and even lighter or no line for background. This can give the illusion of depth. Overall, thin lines or thick lines tend to look more impressive than a middle weight line which can be boring.
Lastly, and this for me is the important thing, your hand has to move fast or fluidly. If you "trace" your original drawing underneath, the line work is often not as expressive or crisp as it could be and often has a halting quality. If you're working over pencils or a lower layer, when you do your finishes don't trace, redraw the image.