Just nicked this from another forum so....
Sandbag Recipe
2 cups of flour
1 cup of cold water
1 cup of salt
Mix flour and salt together. Gradually add the water to make a smooth yet medium stiff dough. Add food colouring, to pre-colour the dough. Add two tablespoons of white glue to further toughen the clay. Form into desired shapes and bake at 170° Centigrade (325° Fahrenheit) for several hours until dry and hard. Paint and drybrush as desired.
Some pictures on how they look.
http://www.miniatures.de/scenery/sandba ... ydough.jpg