EE has a great loyalty scheme for their PAYG element, every three months you can add on an extra 500mb of data, which is appended to each subsequent pack purchased. After a year, this equates to an extra 2gb each time you buy a pack, it's a great incentive to stay. I know a lot of people who complain about the network coverage, but stay only because they do not wish to lose the data they have accrued.
I wondered if Giffgaff could implement something similar, to reward their long standing customers? Perhaps allowing customers an additional 500mb of data per full year of service, to be added on to any goody bag purchased at £10, or above? So, after two full years with giffgaff, instead of getting 3gb for £10, they would get 4gb, after four years, 5gb and so on.
This kind of system seems effective to avoid customers jumping ship in search of a better deal. Just a thought.