Next time you are tempted by a free trial, ask yourself what's in it for the company. If you hand over credit or debit card details to cover P&P for your so-called free gift - there's your answer.
When said product arrives you are distinctly underwhelmed and when another turns up next month you realise you've accidentally signed up for regular payments to be taken from your card.
The agreement is called a continuous payment authority; in theory you can cancel it by contacting the company and asking them to cancel. But you may also need to contact your credit card issuer and cancel directly - this is useful if you can't get hold of the company running the offer. Don't be fobbed off if the card company claims you have to cancel via the comapny supplying the offer. You don't. Once instructed, the card issuer must stop payments - check your statement to ensure it's been cancelled.
Good Housekeeping, May 2014