When captured in such a dense crowd where you cannot even turn around - start breathing deeper, moving your chest up and down, instead of usual shallow diaphragm breathing. Try to keep the lungs half full all the time. Keep one of your palms locked over tightly closed fist under the chin, pressing the fist at the clavicles, and spread your elbows out a little at the chest level, like making a cage for your chest. Avoid spreading your legs too much while walking pinched between people, your leg might be captured tightly. If about to fall over something or somebody on the ground - use elbows to hang over the neighboring people and rise both legs over the obstacle, or step on it. If cannot withstand the pressure anymore push into others in perpendicular to the "crowd wave" direction (usually it goes along the "to the stage" or "to the exit" line), or even try to climb up a bit on your elbows, you are thinner down the legs, and people most likely will try to knock you off back, giving you some space for that. Been there done that.