Have an existing credit card balance which you want to clear
You need to think about how much you can afford to pay off each month. If you decide to pay the minimum then you're hardly making a dent in what you've borrowed and the balance will remain almost unchanged. The more you can pay the better.
For example, if you have a £2000 balance on your existing card with a standard APR of 14.9% and transfer to a card offering 0% for 9 months you could reduce that balance by £630 and save £202.55 by paying £70.00 per month.
Bear in mind that if you make purchases during the 9 month period that standard rate would apply and you would need to increase payments to cover the debt. Obviously the idea here is that you change credit card every 6 - 9 months to take advantage of these offers. If you think you may find yourself forgetting to change then an alternative may be a card which guarantees a low rate for the life of the balance.