Complaining to a company that you feel has done your wrong by providing and inferior service or product to the one advertised and paid for can be a duelling task.
The key to remaining sane during this time is to always keep your inner calm, even if you need to be outwardly push of commanding. Stay calm inside so that you don't lose your head and forget what it is you are really after.
Put your complaint in writing. Whether it be a letter dropped off with their office or an email keep all of your communications in writing so that they cannot backtrack on something they or you have already said.
Once you have placed your complaint give them some time to follow up on the matter on their side, contact who they need to contact and then get back to you.
You can, however, follow up on a daily basis to make sure your complaint has been received and is being actioned. Remember that the bigger the company the longer it will take to sort out. Try and deal with the same person every time instead of being passed from person to person and having to re-explain yourself each time.
If they do not come back to your with an answer that you are happy with don't freak out instantly. This is the time to ask to speak to a manager. If they have some lame excuse regarding why the manager is not available tell them you are not leaving this until you have spoken to a manager.
On that note it is much easier dealing with a complaint in-store. Going into the store or office and talking directly to the manager is going to be a lot easier than dealing with call centres, help lines or till staff that really don't have the power to say yes or no to you. Dealing with those people is only going to waste your time.
If there is a customer complaints department at the company you are dealing with then that would be the right way to go at first because those people are trained to assist you in the best way they can. If you get nowhere with them then take it to a manager.
Never shout or swear. Remain professional at all times and keep communications open otherwise you are going to get nowhere. Keep yourself calm and listen to what they are saying if you expect them to do the same for you.