No sharing clipper cards - they have fare checkers on the trains sometimes. Also you can stop at Walgreens which are everywhere downtown - sometimes within blocks of each other and add money that way (or buy a clipper card there). There's one basically across the street (+ a few doorfronts to the left) from the Cable Car at Powell. There's another one 3-4 blocks up on Powell. The Muni/Bart at Powell station always has homeless people trying to sell clipper cards to tourists (Don't - they probably have a negative value so you just end up putting more money in it or just don't work.) Walgreens is also a good place to get cash back - buy a bottle of water and get $5 back. That's what I do instead of an ATM or going in a branch if I need a smaller bills. Over at the California Street cable car - there's a Walgreens near it too.