Here are examples of my rock art along with some of what I have learned.

I use acrylic paint after outlining the drawing using chalk or coloured pencil.

Clean the rock first with a brush, soap and water so the paint will adhere.
This paint dries fast, which is good when you want to apply thins layers of paint (washes)
over each other. Acrylics are quite durable and easy to work with. They clean with soap and water.

The final results look best when choosing a rock that suggests a shape or scene.
Look for groves, hard edges and contours in the rock to naturally create the effects you would
use shading and tonal techniques with if you were using pencil or paints.

Leaving areas of the rock unpainted utilized the natural beauty of the stone.

Rock painting is a great way to learn, as it is easy to erase mistakes with a wet cloth or cotton swab.

Lin Wellford has published many good books on painting with rocks.
She maintains a web site with many examples of her work and has a newletter page devoted to the topic and sharing other's rock art. Check her out at http://linwellford.com/