I used to tip 15% when I ordered Thai food delivery. One time the dude came within 15 minutes of me ordering (usually takes 40 minutes even though the place is 4 long blocks away) so I gave him 30% tip. I guess the driver was very happy with that, and now everytime I order in or eat at the restaurant, the owner always greets me and I get free appetizers and dessert -- which I accept graciously and give 30% tip everytime.
The gift that keeps on giving 🙂