Finding the best price is a real balancing act. You don't want to wait too long because cabin selection reduces your chances of getting the cabin and or category you require. I usually go ahead and book at a price I think is fair and then watch pricing. If there is a reduction in price, I call my travel agent and ask for the reduction. If it is past the final payment, often they will give you an upgrade or on board credit which is just as good as cash.
Also plane fares are often cheaper when purchased quite a bit ahead. The tickets often do not allow any changes. It is possible to wait for the cruise fare to come down only to find that your plane fair has gone up!
My cruise last June to Alaska on the Amsterdam I booked in March. The price only went up after I booked. In fact it more than doubled in cost between when we booked and when the ship sailed. I'm going to be on the Veendam for Thanksgiving and we paid $657 p.p. for outside cabins back on April 30th when we first booked. The price now for the same cabin is over $1050 p.p.
I guess my point is that waiting is not always in your best interest, especially if you find a T.A. that is willing to work for you in the event of a price reduction.