Let's start with the first and foremost tip...
Be the Boss
You're the one watching them, therefore you make the rules. Don't let them control you. You control them.
Counting to 3
When you are watching them, if they are doing something they're not supposed to do, call them down on it. If they still don't listen, tell them they've got until you count to 3 (or whatever number you choose) to stop doing what they're doing, or they will go in time out (or whatever punishment you choose). For example, last night when I was babysitting two boys (4 and 2) and two girls (9 and 7) The 4 year old was playing the Wii and it was time for he and his brother to have a bath. When I told him that it was that time, he said a flat out no and continued playing the Wii. However, when I told him he had until I counted to 3 or else he'd go in time out, he hopped up, turned the TV off, and listened.
We all know that kids can be extremely frustrating sometimes. And when they are frustrating, they most likely will not want to listen to you. But, you need to be firm yet remain calm. If you aren't firm, they will not respect you and they will know that they can get away with lots of things. If you don't remain calm and you're yelling at them or talking in an angry voice, they will become much more upset and perhaps become a bit scared. If you use the method of being firm but staying calm at the same time, they will listen and they will calm down faster.
Play with Them
Kids won't like it if you don't play with them. Don't know what to do? You can do things like taking them outside to get some energy out, letting them cook or bake something with you, playing a memory card game, playing 'House' (where you pretend that they are people like the Mom, Dad, daughter, son, baby, Grandma, Grandpa, etc.).
Don't Ever Listen to the Kid about Their Bedtime
This is a very, very, very important thing! I made the mistake of listening to the 9 year old about their bedtime last night. She said they all go to bed at 9:30. Mommy says they go to bed at 8:30 and the girls can read for a little while. Oops!
Start with with Two
When it's bedtime for the kids, and if you have four or more of them, it's easiest if you do two at a time. If you're saying "Girl #1 go to bed!" "Girl #2 to your room!" "Boy #1 start climbing up that ladder and into your bed!" and you're maybe trying to get a toddler in his crib, it won't work. Simply start with two kids. I started with the boys since they share a room and then ended with the girls.
Thanks for reading Babysitting For Dummies!