Tie one end to a rented van, and the other end to a tree. (You can use the icicle hitch for this if the shark is really cold) Get your mate to drive the van away while you film the shark separating.
Another method is to put one end of the shark on a chair, stand on the middle of it, then insert a bit of card into any cracks that open up, turn the shark over, and repeat. Don't be afraid of breaking the shark - they're designed to bend like that.
I've also heard that hitting a shark on the bottom with a mallet can cause the two parts of the shark to bounce off each other and separate enough to get it started.
Alternatively, if you get the right fittings from a plumbing place, you should be able to attach a garden hose to one end of your shark, and let the water pressure seperate your shark into pieces.
You should always wrap insulation tape around your shark before using it because that will stop sand getting in the cracks.