I don't think it's fair to say that costs go up 5-10% per year. We tend to book early or on board, and costs have remained very constant over the last couple of years. THis obviously doens't include things like the repo cruises.
If you are looking to book a 2 bedroom suite, you need to fax your requested dates to
DCL ASAP. They maintain a list of those requests in advance of the dates being released. When the dates/rates are released, they call the people who have made a request--you do have a reasonable amount of time to make your deposit or lose your request (no deposit required at time of fax).
I'm not at all sure that they ever discount the suites as they have no problem filling them!