My dad was a manager at a McDonalds where he grew up. Some words of wisdom from him:
1. Don't let the customers push you around. They are paying for their food, not to torture you and make you their servant.
2. The customer is not always right and should not always get whatever they demand and think they should. Story: My dad had a guy come up to the counter and bring back a bite of a Big Mac, all that was left (he at 90% of the burger), and claimed that it didn't taste right and wanted his money back. My dad simply smiled at him and told him that because he ate more than half of it he would not get his money back but get a coupon for a free Big Mac the next time he came in. The customer was mad, but my dad's supervisor was very happy with my dad.
3. Just be polite no matter what you say. As long as you smile and look confident things will go smoother.
4. Just have fun

and try not to get too stressed over the little things. That will just make your job 10x harder.