It's not that hard you know
When painting an area in a base colour, just add a little white in the mix with your brush, and trace the higher lines and edges. Draw neat lines, but not too thin. You can use a wider brush. Then add more white, and mix the paint. Use a small brush and follow the same lines again. You can use the side of a brush on edges to make it easier.
After highlighting, use washes to accentuate the recesses. There are different colours of wash available, but you can create washes yourself with water and a colour mixed with a bit of black paint or black wash.
I've also had great results with QuickShade.