How much do you tip your cab driver?

As you know, I just got back from San Francisco. And while I was there, I took a couple of cab rides. But I have always been curious as to how much you should tip a cab driver. What's the standard? I can only think of one circumstance where I refuse to tip a cab driver and that is if they talk on their cell phone from the moment they pick me up until we reach our destination... especially if I am trying to keep a conversation going with a friend. On the other hand, if a cabbie has been helpful like letting me know where the good strip clubs are or which great restaurants I should check out, I will take care of them - sometimes as much as 100 percent tip. So how do you tip your cab driver?