Nebari is the japanese word for the surface roots of a bonsai tree. This is a very important part of growing a bonsai tree. A nice firm, thick and radial nebari is considered a good trait of real bonsai. Not only does a nebari create a visually appealing bonsai, it will also create stability for the tree.
Techniques for improving nebari
Improving the nebari of your bonsai has everything to do with the depth of your roots. By trimming the vertical growing roots regularly you can stimulate growth of the roots that lie closer to the surface (or even on top of your soil). When you repot (and root prune) your bonsai, you can also reposition the radial surface roots. Look for a nice spread of roots and try not to burry them with too much soil.
A different, and more radical, way to create (or force) nebari is to create a tourniquet. With this technique you use a hard wire at the base of the trunk, which will obstruct the normal flow of nutrients along the trunk. If applied properly, new roots wil emerge above the tourniquet. These roots will eventually form the new surface roots, or nebari.