Stay in a ball for ten seconds. Both the heat and the blast wave will pass over you in that time. Then, if you can, stand up. You will be among the survivors — provide you can move fast enough to avoid falling rubble and fire.
If the explosion catches you one step from a tree-trunk or doorway, you can take that step and crouch with your back to the wall. Ib this position debris and fallen glass will fall beond you.