One tactic I used once is sailing day booking. This of course would work for sailings from cruise hubs (I tried Miami) and when travel plan is flexible.

I was able to save over 40% when booking (directly on cruise line 800 no) on the sailing day and I got that in the form of multiple upgrades and some fee waiver and freebies. On that weekend there were 6 sailings but I couldn't get availability on the top ones (newest ship of RC and PC) but I was able to go on Celebrity Cruise.

I was on a 3-week holiday in Florida and I had very flexible accommodation/car bookings and it didn't matter to me which week is to go for a 7-day cruise.