I'll echo a bit what others said on making sure it's at least a week. I'd also avoid any time in March or April due to college & school spring break. The early part of it can turn any ship into a party ship, and the latter part of it will have at least a 1:1 ratio of kids to adults making for a different kind of hectic cruise. I personally like cruising the Caribbean in late May/early June as the schools haven't let out yet, and you get some really warm weather and ocean temps which make for great days on the beach.
Also consider what line you'll be sailing on. Of the big 3, i