Best route is to develop a relationship with a travel agent you trust. Good TAs will often share part of their commission and purchase on-board-credit. Such OBC can be converted to cash or credit to card if not used. Also: OBC is often part of the ship lines' promotional effort, so watch for same.
Concur with @Allen2 , but I wouldn't emphasize OBC. Instead, I would be selective in picking a cruise that is fairly priced. As Matt and others have discussed, this involves looking farther out when schedules are first announced, considering older ships, and considering less expensive ports. When you have a good/fair price for your cruise, an extra $50 here or there really doesn't make a difference. But by purchasing the cruise with a credit card, you end up getting points of some sort that can be redeemed for OBC or cash, depending upon your bank. I use Royal's Visa through BofA but other passengers have found great deals with the cards they use.
I have no problem comparing cruise prices (across carriers) online before I talk to my agent. Just make sure you are making apples to apples comparison (is the fare before or after taxes and port fees, for example?).
A good agent can help with many complications. Knowing where your hotel is located the night before your cruise is important, and a knowledgeable agent can set you straight on that. We always use an agent.
Whatever you choose, have a great cruise.
