They don't really offer discounts. They have offered incentives in the past, such as a Visa Gift Card or a 3-day
DCL cruise. The incentives are only available for specific itineraries and dates (the ones that aren't booking well enough), so there is no guarantee there will be any at all. Some travel agents offer incentives, as well, like the DU offer that's in the announcements on the top of the main page of this forum.