How do I make a formal complaint?

In Feb I chatted with an ATT rep regarding my cable/internet bill which had slowly soared to almost $200 a month. I told her I wasn't willing to pay that price and asked if there were any promotions. She said yes, there are, and she can apply one that will take my bill down to $79 a month plus taxes and fees and no change to my existing service and I said sure! She stated that she would apply these changes right away and that my March bill would reflect the new rate. She gave me a confirmation number and ended the chat. Fast forward a month and my bill comes and it is again $200. I chatted with another rep who was surprised I had been given that rate (I had saved the original chat and copied and pasted it for her to see). Apparently the first rep had no business giving me that rate, did not apply it to my account, and did not put notes about what our chat contained. ATT does not save chats, luckily I did. There was nothing the rep could do for me. I called customer service, and again, nothing they could do besides take my bill down to $139 a month. It seems to have escaped their minds that if their employee screwed up royally, they should bite the bullet and honor what the customer was promised. I am so angry and hurt by their lack of customer service, I cancelled cable today and will be cancelling internet as soon as we move in 2 months. I would like to file a complaint up the chain to make sure management knows about this ridiculous situation in which a customer was wronged and nothing was done to fix it. I can't seem to find a place I can file any type of complaint. Any help? Any other avenues I can go to to make sure this doesn't happen to anyone else? It is safe to say I will NEVER use ATT again, they are losing out on way more money losing a customer for life than they would have if they just fixed this. They will also lose my family and friends as customers because I have passed this onto them and they have started cancelling service as well. I did file a complaint with the FTC so if there is a trend they can investigate it. ATT has been majorly dinged by them before, I am sure it could happen again.