Early bookers get the best discounts. As cruises for Jan10 have been on sale for a few months now you will already have missed the lowest prices. If you book with RCI or Celebrity and the price drops from what you paid then you will get the lower price - so with them it is a win-win situation. Alternatively you can leave it to the very last minute when the prices can really fall if there are many rooms left, but then you will pay more for the flights so you don't really win that way either. If it is a special birthday and you are tied to a certain date then I would book asap.
Totally agree if you fancy the Caribbean then Jan is one of the cheapest times to go and the weather is very nice,try and price it on there web sites then try a cruise agent for the best deals,best of luck?
You don't mention if their is a specific date that you wish to be on your cruise on, and you don't say if you have cruised before, or what your budget is? It sounds to me though that you should be seriously looking at booking now! Most of the decent Caribbean cruises are costing around £100 a day at the moment for an inside cabin, which is a fair price to pay considering flights are included in this. Although sometimes the prices of a particular cabin may drop if you leave it later to book, the actual type of, or position of the cabin that you wish to book may sell out. Not much good having a half price suite available if you can only afford an inside cabin! Before eventually choosing your cruise, check out who you will be flying with also. Some charter airlines cram people in like sardines, and this can be very uncomfortable on a 9 hour flight there and back! Happy cruising.