Highlighting is actually a simple thing, as I recently discovered. Washes gives your model depth with dark colors; highlighting raises parts of the model with lighter colors.
Simply take a lighter shade and edge the natural lines on your model. The lighter shade can be a couple drops of white in your mix. This sort of painting also requires a tiny brush.
This Space Marines model has clear examples of highlighting. The edges of the shoulder pads, bolter, top of the knees...
You don't have to highlight, but you'll be glad you tried it. It creates a better contrast between colors and gives the appearance that the model is catching light.
