Unless I start going to sit down restaurants a ton (I'm much more likely to get fast food), I just default to 20%, and you're probably going to have to piss me off for me to lower it.
I tend to forget that other jobs ask for tips. I don't get haircuts often, but when I do, I look at the tip line on the receipt, and I'm like, "uhhhhhhhhhh" and then just decide, "two dollars, why not", or "five dollars, why not," or whatever, depending on how much the service was.