Here's my take on it.
Cruise Lines - I've only been on Royal Caribbean, Carnival, and Norwegian Cruise Line, so I can only compare those 3. My favorite cruise line is Royal Caribbean. More mature, a little more upscale than Carnival. Carnival is ok for a younger crowd or younger kids. If you have children, Disney!!! It's more expensive, but tons of stuff for the kids to do. They take the kids for the day, so the parents can relax.
Room - When picking your room, I try to get as close as possible to the stairs or elevators. Get an inside room, as cheap as possible. The less you spend on the view, the more you can spend elsewhere. You can do a balcony room on a 3 day cruise, the same price for a 5 day inside room. I'd rather have a longer cruise than nicer room. I've gone on so many, the view from the room doesn't matter to me. I like sleeping whenever I want (14 to 16hrs a day on a cruise) and if you turn off the lights... COMPLETE darkness. If your not asleep or involved in some kind of bedroom olympics
EDIT: (sexy time, hanky panky, there's a party in my pants and my wifes invited), you should be out of your room enjoying the boat, views form the top deck, gambling, watching a show, in the pool, at the spa, etc.
Packing - On board, I only wear bathing suits, t-shirt, and flip flops. I'll take 5-7 board shorts and surfer type tee shirts. I'm on vacation, I RELAX !!! Of course I bring decent clothes for dinners. I ALWAYS bring an extra duffel bag or I double up a piece of luggage (smaller piece inside a larger piece) because of the stuff we buy. If you get on board with your suitcases full, you'll have NO ROOM for the stuff you buy.
Drinks - My wife and myself don't really drink alcohol, so we save a ton of money there. Buy a soft drink package when you first get on board. It's a set amount of money (so much $ per each day of the cruise) and it gets you unlimited soft drinks. You only need 1, because we share the drink. We usually designate 1 night to get drunk, and just buy from the bar. I have friends who with bring a giant jug of water with them, (holding it in their hand walking on board) HOWEVER, the water bottle is filled with vodka or any other clear liquor. Or a liter of coke, filled with dark liquor.
Money - I'll bring about $100 per port to spend on stupid crap, gifts, souvenir, local food, etc. I also bring about $100 per day to gamble on board. Even though you can take money from your room account (from your CC) I like to have the cash, so I know how much I can loose
I've been cruising for 7 years and we go on 1 to 2 cruises per year. I've been almost everywhere in the Caribbean, and last year, we did a couple weeks on a cruise in the Mediterranean (Italy, France, Spain, Croatia).
I'll post more when I think about it.