You can book at the prevailing rate if there are cabins available. No special discount but sometimes last minute bookings are cheaper. Quite a few people were doing this last year when ships weren't full - and they got some great deals. And if you book onboard, you do get a slight bump up (inside, OV, balcony only) so a better cabin. You can also try to negotiate CruiseNext certificates, although those purchases aren't usually processed until after a cruise.
Some of the CruiseNext folks are quite creative, some are relatively clueless, so you might have to make suggestions, based on replies here, to get the best value. Good luck!