I went to lunch today and I think the bill was like $30 or so, so I tipped $8. That chick was awesome. I pretty much always double the tax, and if the server was particularly cool, I'll add on to it.
^ Pretty much this.
I order from the same pizza joint every friday night. I always tip the driver extremely well, like $10 on a $20 order. No matter how busy they are, I always get my pizza in 20 minutes or less. And it's usually loaded with toppings and hot & delicious every time. Trust me, people take notice of good tippers. You will definitely get better service as a good tipper in restaurants and establishments you frequent.