I always tip at a sit-down restaurant where you are getting served, the amount depends on the service (you must have really annoyed me to get less than 20% in most cases, though ). Picking up food or fast-food? Never. Delivery gets a minimum of $5-6 tip, 10-12% otherwise.
The one thing, however, is that I refuse to tip tax. Meaning, I tip based off the pre-tax amount on the bill.