Dealing with water systems over the past few years I have found a great deal of reasons for higher than normal water bills. Leaking toilets are the #1 reasons for higher than normal water bills. Take the top off of the back tank of your toilet and listed for a hiss or drip. If you hear any noises while the toilet should be quiet you might want to look into replacing the seals in the tank. Kits are only around $15 and can save you hundreds. These leaking toilets are leaking 24 hours a day so the amount of water wasted adds up quickly.
If you have a softner system and/or a humidifier attached to your furnace make sure you have these systems checked. If softner systems are not properly maintained they will act up and use a large amount of water.
Humidifiers on furnances add water into the heat as it goes into your ducts. This will also cause your bill to increase depending on what setting you have it at.
Make sure your faucets on the outside of your house are not dripping. I believe most of your meters are located on the outside of your house in a pit. If you have a meter inside your house, you can look at the dial and see if it is moving. When you arent using water nothing should be moving on the face of the meter. ( The part that has the numbers)
As I think of more ideas I will let you know.
Bryan Rhoads