If you can swing it, go during the off season. Look at the prices and you’ll easily see that the off season is MUCH cheaper!
If you are able to plan in advance, then do it. Get the time off work that you need and just book it.
Disney offers discounts to military personnel from time to time. There’s almost always a deal for military personnel, it’s just a matter of what the deal is.
Before you book your Disney Cruise, take the time to research all the ships. You may find that the more classic ships are cheaper, but very worth your time.
If you aren’t going on your cruise for a year or so, make payments on your trip! You do have to make the down payment, but from there you can pay as much as you’d like.