Trying to save up
Or just ask nicely in this thread and some kind soul will often gift you some.
Sometimes Fizz will offer perks you can activate for free data.
Also participation in the forum will give you points that help you level-up. Higher level means more upgrades (a lot of them are data upgrades. I have 1GB for free each month for example).
As other members have mention, there are 2 ways to get free data