Just fill your mouth with toothpaste and droool ....
Or slip an Alka-Seltzer tablet in your mouth.
Back in the late '50s or early '60s my younger brother and I were on a NYC subway. He did that and then fell on the floor. Cleared the car in no time. Scared Hell out of me as I thought he was really having a fit - until he started laughing. We got off at the next stop and high-tailed it.