READ YOUR BILLS
A lot of people receive a bill, look at the amount owed, pay it and toss it to the side. This is a bad habit. I can't tell you how many times I've found weird charges on my bill or I haven't received promised credits or promotions on my bill. It's important to keep track of all the expenses on your bill because if you read the small print many companies give you a time limit in which you can dispute charges after the fact.
It's also a good idea to read the side columns for information on current promotions or upcoming changes. Having worked for a few different call centers in my time its amazing how many people don't read their bills and then call in and complain when they see a charge for something has increased. For example, the price of calling 411. If you don't read your bill from top to bottom you could be costing yourself more than you realize and companies typically don't credit back for this. If you get a good agent they may credit you the first time but you can bet they will notate your account so that you don't get that credit again. If its a big charge you may only get a partial credit. Since they informed you in writing (your bill) they technically don't have to do anything.
The point is it's your responsibility to check that all charges are accurate and that you are up to date with fees associated with your usage. So get out your magnifying glasses and get informed!