Here's a LifeHacker.com worthy tip. Credit goes to my friend Julie for sharing it with me the day we went to watch a movie and their credit card machine was busted:
Put a small amount of cash in your car's glove box... just in case. Just in case the movie's credit card machine breaks. Just in case you find yourself unexpectedly crossing a toll bridge. Basically, for those times you need cash because for whatever reason, the dream of a cashless society came crashing down on you.
I put $11 in each vehicle. One $5 bill. Five $1 bills. And four quarters. Enough to cover 99% of the situations I find myself in, but not enough to worry about it being stolen (but seriously, don't steal from me, friends).
When I use the cash, I leave myself a note on RememberTheMilk.com to refresh it the next time I visit the bank.
There you go. A simple tip to get you out of a plastic pinch.