Consider getting a TransLink card to ease your daily commute.
The TransLink card is now available for use on Muni, AC Transit, Dumbarton Express, and Golden Gate Transit and Ferry. TransLink is not currently accepted on BART or CalTrain, but eventually will be used on all transit systems in the nine-county Bay Area.
Your TransLink card carries a pre-paid balance, so you can forget about scrounging in the sofa cushions for bus money, and it provides protection of your balance if your card is lost, stolen or damaged. There is no fee for a TransLink card if you set up Autoload at the time you order the card online.
You can order a TransLink card here: https://www.translink.org/TranslinkWeb/getTranslinkOrderCard.do
...and don't let this be you!