Very good advice above, but sometimes there are exceptions
Last week, booked the Liberty of the Seas 4/8 for 12 nights FLL-BCN. The doubles price was down to $369, and the single supplement was only $159, or $528 for the cabin. If you belong to Crown&Anchor, there was also a savings certificate for $115 off the price, bringing it down to $413. Apply a future cruise deposit and they added a $150 OBC as well. If you're a Diamond or higher level, they throw in 3.5 happy hours every night - that's a value I can't pass up.
However, if you snooze, you lose. I check almost every morning for the deals, and limit myself to just two cruise lines nowadays - it still takes me an hour plus.