You need to always check the bill. Perfect example is today when I reviewed my AMEX bill and noticed that the Wall St Journal charged my card for the entire year subscription. They had me setup as automatic renewal, so I was never contacted about it. I receive this through my work also, so there is no need to pay for it twice. I immediately called them and cancelled it. It ended up saving me $265.
Also, you need to look for fraudulent charges. Most thieves will steal your identity and run up huge charges. But some are getting smarter. They'll look at your bill and see where you're spending money and then go to those same stores and charge smaller amounts that you would never even think about. This can go on for years without it ever being realized.
If there's ever a charge that you don't recognize than don't hesitate to call that company to find out what it was for!