Bang it against a wall and hose it down.

That’s what I do. Shaking alone leaves sand trapped in the weave, especially if the bag’s damp. Take it outside, slap it hard against a fence or wall a few times, then rinse with a hose or in the shower. Let it dry fully before packing away. If you’re at the beach with no hose, just keep whacking it until no more sand falls – then whack it three more times.