How Do They Do That?
Shoveling:
Every grain of sand that you see in your favorite sculpture was shoveled there by its respective sculptor. Forms are made of 5/8" plywood and two-by-fours and sand from the beach is shoveled in to them with plenty of water to make it workable.

Tamping:
After the sand and water combination has been placed into the forms, tamping is required to compact the grains of sand and get the square surfaces lined up to each other. This is usually done with a heavy metal tool that the worker pounds onto the surface until the next layer of sand/water is applied.
Water:
Without water, the dry sand would just slump to the ground and be impossible to work with. You may think of it as the 'glue' that holds all those wonderful creations together. Water is delivered either by hose (supplied by the Village) or by bucket. Can you guess which way is easier?

Carving:
Once the forms are complete and the sand is compacted, the sculpting can begin. Work is done from the top down with the forms being removed when they are no longer needed to stand on. When all of the carving is completed and the forms are removed, the focus is on the ground area. Many artists spend considerable time with elaborate detail on the grounds, adding to the wonderful effect that makes attending Harrisand a truly memorable event.
Plastic buckets often come in handy for small, specific shapes that the artist needs.
Not everyone uses wood or plastic forms. Here is a technique where very wet sand is stacked to form the desired shape for carving.
Among the many different tools used by the sculptors, drinking straws are great for adding some amazing detail where a knife would be too clumsy
Click HERE for a full text version that goes into much more detail.

Further reading::
National Post article on The Physics of Making Castles Out of Sand.