Omit your line layer or paint over the lines. Leaving a hint of them there might work, if they look like they were traditionally painted on (usually much easier to do if they're thick and globby).
Try thinking of your tool as a paintbrush. Have you ever painted traditionally? What with?
My experimentation is limited, so I haven't much messed with custom made brushes that attempt to emulate traditional ones. For technique itself, using a higher opacity and a hard brush tends to lead an image to look like it's been done with acrylic... it kind of feels more like using acrylic too, ultimately thicker and not as smooth as oil...
In this picture, the chain in the midground looks very digital, due to the completely solid completely opaque round brush used. The edges are just too crisp. The one in the background has a couple areas like that too.
The one in the front is pretty good but I don't have much of an eye for having blurry acrylic... hyper-tom's really good at it though, but he uses an airbrush. http://hyper-tom.deviantart.com/art/Deja-Vu-89307604
For the character, maybe work on blocking in the area instead of relying on lines to convey the edges. Paint the form from the silhouette.
Always use the biggest possible brush for rendering, and add the sweet details in last, including any useage of black or white.
Limited brush experimentation:
http://fchan.us/artist/res/1110.html
The deer guy at 1125, pretty much all experimentation, I used textured brushes. Some parts of the trombone really stand out. I don't think I went under %50 opacity at all.
Uhhh.
This isn't really organized, for which I apologize. Hopefully something makes sense and/or works, but if it's all nonsensical I can try to clarify.