When you're wrapping wool around a borb base, or adding layers of wool to a piece, as much as possible try to preserve the fibers going in the same direction. This will give a uniform look to your piece, especially where all of the same color fiber is used. It's not always possible depending on the shape of the piece, of course. When possible, avoid making the fibers on your piece of the same color or detail overlap perpendicularly.
(If you want to create details that are visually separate, like a bird's striking cap that pops out against its back color, of course placing the fiber in an opposing direction will give the desired effect.)
In the image on the bottom, I have placed fibers on an incomplete borb's head which run both askew and perpendicular to the fibers already on the borb's chest. It looks more polished in the images above where the fibers all run in the same direction on head and chest.
You can also blend wool fiber colors by placing light colored wool on darker wool, and vice versa. For the best effect, again preserve the direction the fibers are running in.