Every time you want to buy something go through the following process:

Do I really need it?

        If 'NO' then don't buy it
        If 'YES' then ask:

            Can I buy it second hand?

                    If 'YES' then do so
                    If 'NO' then ask:

                         Can I buy it from a local producer (and preferably organic)?

                                 If 'YES' then do so
                                 If 'NO' then ask:

                                        Can I buy it fair trade?

Most people own too much stuff. Simplifying your lifestyle is an effective way to make a difference.