I start with a base coat of Chaos Black. Along the edges of the surface to be highlighted I paint successive bands of a thinned down mix of Chaos Black and Skull White. The first band is a very dark grey and every successive band is a little lighter (more Skull White) and a little thinner than the band which came before. The band along the edge of the armor is the thinnest and the lightest grey. For table-top models I normally paint three or four levels (bands) of highlighting.
If you are satisfied with the result then stop here. If not, you can take it a step further.
Once I am finished highlighting, I mix a very thin wash of Chaos Black and water. It is important to mix the wash thin because if it is too thick, then all of your hard work will be covered up. The wash is applied to the highlighted areas. Take care not to let the wash pool on any flat surfaces as it will produce areas of dark splotches over what should be a smooth graduation from dark to light. Let the wash dry completely. If you are unhappy with the results, apply a second coat.
If the wash produces dark splotches or does not produce a consistent graduation of color across the highlighted area, mix-to-match some thinned down Chaos Black and Skull White and touch up those areas by hand.
Here's an example of the finished product:
http://i72.photobucket.com/albums/i200/10011970/scsq1.jpg