The guys are on the right track here.

Scout around for more clayey soil. Dig a pit (cauldron) in the clay. Fill with water. Heat rocks, use tree limbs to handle hot rocks into water. Steam is generated. Hold shirt over steam. Wring out shirt (distilled water) into mouth or container.

Downside:
Need many rocks, large long lasting fire.
Need rocks that won't explode when heated.
Need head examined for going out without correct gear. <img src="/images/graemlins/grin.gif" alt="" />

(Tip from Australian Army Survival Wing)