Princess has a pretty good program, called Captain's Circle. I'm not sure it's good enough to make you cruise Princess just to work your way up. The best way to keep people coming back is to offer a good product--a good value for the dollar--and that is what keeps me coming back. But the rewards certainly don't hurt, and are kind of a nice little "thank you".
Princess recently modified the program slightly to benefit those that take longer cruises, which includes a good percentge of their clientele. After one cruise you are a Gold Member. This gives you some inconsequential perks like Princess Passport (a little book in which to keep souvenir "stamps" of your cruises), a member benefit card, quarterly newsletter, free photo contest, onboard host, special section in the Princess website to monitor your cruise history, referral rewards (credits for getting others to book), and standby program, which I don't think anyone understands--not even Princess. And then a few worthwhile items like the onboard cocktail party, certain other onboard events, and probably most useful, preferential pricing offers. I do a lot of comparison shopping before I book, and I have saved anywhere from $200.00 pp on a 7 day Mexican Riviera up to $500.00 pp on a 15 day Panama Canal cruise.
Then starting your 6th cruise or after completing 50 cruise days you reach Platinum status. This gives you all the Gold benefits plus: Preferred Check-in, $75.00 to $200.00 pp internet credit (depending on length of cruise), free upgrade to the better Travel Insurance package, Platinum disembarkation lounge, and (wow!) complimentary cruise atlas--which anyone can get free by calling 1-800-PRINCESS.
On your 16th cruise of after completing 150 cruise days you reach the coveted Elite level. This gives you all the previous benefits, plus: afternoon tea served in your cabin (if desired), upgraded stateroom amenities, deluxe canapes when desired, complimentary ONE-TIME mini-bar set-up, complimentary wine tasting, 10% boutique discount, priority tender embarkation (although one of our members had trouble with this one), priority disembarkation, and possibly best of all, complimentary shoe polishing, dry cleaning and laundry services.
Now all of this isin't bad--a very nice way of rewarding loyal customers. But it's not, or shouldn't be a goal in and of itself.
The best way to reward loyal customers is to keep giving them such a memorable vacation that they wouldn't even think of going somewhere else; and Princess needs a little work in this area.