Put simply - we shop deals.
We've found great prices with Royal Caribbean's website, as well as 3 other cruising specialty sites. Many have promos.
We've gotten prepaid gratuities, free spa time, pre-paid specialty dinners, frequent flyer files, and other "promo bonuses" (and/or onboard credit).
In addition, we use Crown & Anchor discount certificates and also watch for price drops so that those can get applied toward our reservations as well.
Bottom line - shop around and do your homework. The pricing can vary hundreds of dollars and the bonuses vary as well.