This follows on from the earlier post about getting a free O2 Money card ( I was able to get mine, thanks!).
If you top-up your O2 Money Card with two top-ups of more than 50 pounds each in 2 calendar months, then o2 will give you 5 pounds for free.
You can just top-it up free of chage and use for your day-to-day purchases instead of your normal debit card. I am sure most people spend more than 100 pounds per months on their debit card quite easily, so this is an easy way to get 5 pounds for free.
Full details here - http://money.o2.co.uk/terms2.html