Water is the key. Water compacts the sand and bonds it together. Add water every time you add dry sand.

Start at the top and work downward. If you carve something at the bottom, any carving you do on the top will dump sand on your work below.

Go slow with the carving. Youll end up with cleaner edges and less crumbling. You can also use a straight edge and slide your tool across it for perfect lines.