What is the best way to clean carpet?

I am decorating my room and want to clean the carpet and remove all the dirt and stains. What should I use to clean it? Are any of those "spray on, vacuum off" cleaners worth using and if not what can I use in place of them?

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Advice No.1 - Do not use supermarket carpet cleaning products. No matter what it says on the label - they don't work. Or at least they don't work well enough.

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