Carry over unused data from previous several cycles. This way customers do not need to pay extra on months where data usage is a lot while in fact there are some months where data usage is not even half the limit.
This is really a great idea. I just checked my bill for last month. I paid $5 for the only over-limited ~0.5MB. I hope the rest data usage, which I have already paid for, could be carried-on to the next month at least.