Do not leave till the last minute,although you may be lucky,we book about 6/8 months ahead,check the daily mail on saterdays for ads,for a particular cruise and wait till you see what you want advertized,gills cruise advertizement is a good one for price,but we use travel councellors based in bolton you can find there number off teletext they usualy have a page on ch4/5.both these agents are good for price.
From the prices I have seen for next year and compared to the price we have just paid for 2 weeks on the Thomson Celebration I would say its best to book last minute. I booked 26 july departing 3rd August. But only if you are happy with a lower deck inside cabin as there tends to be little else left, but how much time do you expect to spend in your cabin?? Also with last minute you lose the option of drinks packages but I think they are overpriced anyway.
Depends what you're happy with...
Do you want a specific cruise/ship/deck/grade/cabin, if so then you'd be advised to book early, there are substantial discounts for doing that.
If you're pretty easy about what and where then you can take your chances and book late.
Remember, the couple in the next cabin that booked at the last minute may not of been bothered where they went, as long as they had the cruise. You on the other hand may of only wanted that cruise/deck/ship/etc.and not og got that booking last minute..
Having done 19 cruises we found that we have saved by booking even up to 13 months in advance. My wife isn't the best of sailors, but by booking a midships cabin, (which are usually the first to go) any turbulence from a rough sea is greatly minimised. Also the same applies to the flight if going to Barcelona or Miami is involve.
It's a fine line. we track cruises through teletext page 228 & Saturday's Daily Mail. Book early & you can get the cabin you want, but may pay more than booking later. Book too late tough & the ship fills up. Our criteria is to pay only around 50% of the brochure price (get the brochures 1st), whenever the cruise is. The later you leave it, the more likely you will have to put up with flights that are not the original cruise company ones, maybe reasonably direct, but a more round-about route. This has happened to us.