Jetstar has a great deal where you purchase any voucher, and receiver another to 10% of the value. This is an awesome way to save a further 10% off your fares. Trick is to buy just below the value of a fare you have in mind, so you don't forfeit the balance. You can use multiple vouchers to purchase a fare. Remember you also save on credit card booking fees, as Jetstar don't charge fees for payments with vouchers. Good luck!
Link to the voucher page is here.
Buy $50 voucher, get free $5 voucher
Buy $100 voucher, get free $10 voucher
Buy $200 voucher, get free $20 voucher
Buy $300 voucher, get free $30 voucher
Buy $500 voucher, get free $50 voucher
Jetstar Gift Vouchers are valid for only 6 months from issue date