So my wife is "Platinum" and I am "Gold" on DCL and that gives us a little bit of a jump on reserving newly announced cruises. For example, they just released the Summer of 2018 schedule and we booked a stateroom in our favorite category for the Copenhagen - Norway - Iceland - Dover 10-day in August 2018. Gold, then Silver Castaway Club members will be able to book these most-recently announced cruises in a few days, and then the general public shortly after. As most of you know from studying the TouringPlan's FareTracker (R), the prices of Disney Cruises rarely go down, so booking early is often advantageous.
We could call up DCL directly, but we like working with a very competent and helpful Travel Agent, named Amber Vaughan from Small World Vacations. Depending on the cost of the cruise, she/they often will "kick back" some ship-board credit when the cruise actually sales. I have never been a big travel agent person, but using Amber has been very helpful in other ways, and I do not recommend lightly - she's good. A Disney-oriented travel agent would know what day they will announce the Fall 2018 cruises, and when you can book them (we jumped on a Eastbound TransAtlantic in 2008 at a very good price).
There are other "tricks" like booking a "place-holder" reservation while on your cruise that gives one a 10% discount on one's next cruise . . . but this is complicated and my wife understands this better than I do.
Remember, deposits, sually approx 10% of a cruise and are completely refundable. We actually had to cancel a planned Aug'17 Eastbound Canal cruise, and save our money for another day. Also, when I purchase with Disney Visa, there's another 2% discount, and I have 6 months to actually pay that portion of the CCard bill.