1. Steady hands
2. Fine brushes
3. Little paint on your brush
4. use the side of your brush if you can
1. Steady hands, rest one hand on the other as your paint. It's hard to demonstrate. In attached picture, Duncan is resting his painting hands against the model and his pinky and ring finger are resting on his other hands, his elbows are probably on the table. This helps keep your hand steady if you have a habit of trembling.
2. you need a fine brush for a highlight, if you're trying to highlight with a basecoast brush or a 20 brushes for $10 brush then its going to suck balls
3. You do NOT neeed a lot of paint on your brush and what paint you use should be thin. drag a brush across a kitchen towel or toilet paper before applying, your brush should still resemble its original shape even when paint it on.
4. Use the side of brush, not the point, when you can. run the side of the brush along the edge you wish to highlight, not always possible but it can help. See pic related Duncan doing it on a model.