What can I use to make a quicksand scene for a film or a location?

Most common is sawdust and oil. Vermiculite and Fuller's Earth makes a quicksand almost safe enough to eat. Ground cork is one of the best ways, as it leaves the actors relatively clean when they're pulled out. Check out below scene from Perfect Stranger's tv show, using ground cork;

Mud is rarely used...because is looks like...well, mud.