If it's really bad you can pick up a pumice stone in the cleaning aisle of any department store. Scrub the ring with it, The pumice will remove the ring without scratching the porcelain.
You can also pour a can of Coke into the toilet, let sit for a while and scrub clean.
I heard that mixing non-gel toothpaste with Coke makes a really effective cleaner for toilet bowl ring.
Pumice works like Debbie said. To prevent recurring, you can use a denture tablet, a pack of lemon koolaid, or an ounce of bleach. Most people stay away from bleach because of kids & animals.
POUR IN A BOTTLE OF COKE AND LET IT STAND OVERNIGHT