Not a big user of prepaid myself, although I think it has some great benefits over post paid (like possibly rolling over data each month, no inflated charges for exceeding your limit, portability and no Premium SMS fees.)
Optus used to give users free credit for watching ads on their phone, but apparently no more.
FWIW I think the biggest way to save is to stay nimble. Look out for a good deal and get it. Meanwhile keep you eyes out for other good deals (some using the same underlying networks etc). Port you number to that and then repeat. You shouldn't do it too much but moving providers every say months six is pretty painless and a great way to save.