Writing
Good companies will listen
They want to hear your complaints - this is because an unhappy customer often becomes their competitor's customer. You also become the worst walking advert a company can have - telling friends and family of the poor service you have received. However, if they successfully resolve your grievance they will likely keep your custom.
Know what you want
If you have bought a product and it has arrived faulty, you obviously want it exchanged. But have you been inconvenienced? If so, what additional extra will make up for that?
Or perhaps you had a poor stay in a hotel; your room wasn't available when you arrived and the shower wasn't working properly. Why should you pay the full amount? What about a free night's stay, discount or an upgrade?
A fair but firm approach
It's important to remember that it's the companies' product or service that has gone wrong and not necessarily the fault of the person on the end of the line/in front of you; ranting at them rarely gets the best results, so be polite but firm, stating what's wrong and what you want done about it.
An example for returning a faulty mobile phone could be a more conversational version of:
You: 'I have just joined your network, and am disappointed with my first dealings with your company. The phone you have sent me does not work. '
Retailer: 'I'm sorry madam, send it back to us and we will replace it immediately.'
You: 'Thank you, however this is hugely inconvenient as it will be difficult for people to contact me, added to this I will incur costs when using public phones. As a new customer, I am concerned so early in my contract I have a problem. As recompense I would like you to credit my account with 2 months free line rental and guarantee me the phone will arrive within 4 working days.'
Whenever you agree adequate compensation, take a note of the name of the person you spoke to and the date and time you spoke to them.
If at first you don't succeed
If they are unable to meet your requests then politely ask to speak to their manager, sometimes the initial person you speak to has limited authority.
Complaining by letter or email
Follow similar lines to the above, but where you can, get a name of a person to address the letter to, rather than using sir or madam. Make the letter succinct and use any evidence you have to back up your argument; for example, photos of poor workmanship - perhaps send it by recorded delivery and keep a copy yourself.
Remember: the law is on your side
The Sale of Goods Act - http://www.berr.gov.uk/whatwedo/consumers/fact-sheets/page38311.html - details your rights as a consumer when a product is faulty.
Marc Lockley writes for the Guardian and is the author of How To Pay Less For More: the consumer's guide to negotiating the best deals - whatever you are buying - Marc's opinions are his own and for general information only. Always seek independent advice.