As the oldest in a quite large family, I've had my fair share of babysitting experience.  Here are a few ideas that parents, older siblings, friends, and babysitters might use.

How to babysit like a pro

- Board games.  Depending on the age and maturity of the children, there are a variety of different challenging games. 
Chess, Scrabble, Mastermind, Monopoly, Backgammon, Chutes n' Ladders, and Battleships always draw a crowd at our house.

- Movies to watch.  Far superior to TV programming.  You can control the content easily, and the quality tends to be superior. One child is often easy to please, but it may take a little forethought to have something all the siblings will watch.
Look for something that children of different ages can enjoy.  Keep in mind the parents' rules and the children's tastes.
      
       A few family favorites:

- Healthy, non messy snacks. 
Keep in mind whatever you feed to a child you may have to clean up off floors/ceilings/walls/windows.  Dry cereal, fruit, granola bars, pretzels, animal crackers, peanuts, and chips all work nicely.

- Safety First.  Phone numbers for parents, doctors, ambulance, et c. 
    Know where the fire extinguishers and the first aid kit are.  Learn CPR if you babysit often.

- "Child-proofing".   Cleaning up nail polish off furniture isn't my idea of a fun afternoon.  Keep knives, cleaning chemicals, cosmetics, foodstuffs, important papers, electronics, and other accidents waiting to happen out of reach.

- Delegating tasks.  If you are babysitting some older kids too, put them to work.  It'll keep them out of trouble and hopefully get something done.  Laundry, dishes, meal preparation, yardwork, tidying, car washing, -- whatever.

Finally, get to know the kids, use common sense, and have fun!