Muni Karma is a lot like parking karma. It comes and goes on it's own whim. If you don't have any Muni Karma built up you'll find yourself standing at the bus stop waiting for a 33 for half an hour only to find that as soon as you start to walk away one roars by without stopping. One way to build up your Muni Karma is to thank your bus driver. You put your life in their hands every day -- think about how many times you have crossed in front of a bus without thinking twice not to mention how many times you ride in one -- so saying thank you now and again isn't too much to ask.