My lines are pretty much awful, but I just smooth them out with the eraser until I"m happy with them. I guess the best tip though is to avoid scribbling back and forth over the same section trying to find the 'right line': Long, sweeping strokes turn out nicer than short hairy ones ^^
I don't really see anything wrong with your drawings. We all simply have our own styles. Yours is much softer than mine. I've always been the type that first sketches with a pencil and then draws with a marker on top. Very hard and clear lines. The problem with this is that I find it hard to correct and redraw crooked poses and anatomy. It's as if the line made by my pen is somehow magical and my world will come crashing down if I erase it
If the messynes really does bother you, I guess you could try tracing an existing line art piece with the Pen tool and then stroking it with a 4pt hard brush? This is what I did with my "Mean druids" pic because the original line art was done with a 1mm marker (LOL).