I tipped my cab driver $40 today because he not only drove me all over creation, waited for me while I was in the bank and the auto-parts store, and made very pleasant conversation during the drive, but he also changed my dead car's battery for me (the whole reason I was taking a cab around to begin with) without batting an eye.

For stuff like hair stylists, tattoo artists, cab drivers... things like that? I usually do a base 20% and then add to it if they went above and beyond like this guy did today.