Usually I just keep reading with a poker face, but if it's really too much and I am alone, I lay down the phone and hit the closest surface with the base of my fist repeteadly while giggling or saying something like OMG (≧∀≦)
Talking about this, (〜￣△￣)〜 you may be interested in the Wikipedia page about secondhand embarassment ( htt...... reply
