My booking number is 60669562.

After making a booking, you receive an itinerary number. For every 10 nights, get one free! Members can also access secret prices. If you stay book 30 nights a year, you become a gold member! Members can receive a discount of up 10% per booking (the so-called "Special Price".)
On top of that, it is also possible to obtain a voucher code allow even further discounts (through the Price Match Guarantee scheme.)
Point Hacks claims that free accommodation is offered in return for a review. He also claims that you can buy discounted vouchers on CashRewards.

Let's build a formula:
(Assuming I book 100 nights of accommodation per year with
1.1 (free night) * 1.03 (Special Price) * 1.045 (Cashrewards) * 1.01 (Amex Rewards) = 19.5% discount for every night.