Tough question, but like Mike said I find out what the airfare is thru the cruiseline and then search it out on my own. Have found that most times the cruiseline air is cheaper than I can find.
Living here in Florida has given me another option on the Liberty transatlantic however. I booked thru Sidestep.com a one way fare and got it cheaper than the cruiseline was offering. Only kicker is that I have to drive to FLL to fly out and then drive back to Orlando from Miami after the cruise. Still cheaper and the drive isn't that bad.