There's lots that could be offered to such top customers to retain their loyalty. Offer them a complimentary limo ride to/from the airport (even from the ramp if possible on domestic flights so they don't have to mess with the riff-raff in the terminal). Make sure their favourite snack is available in the MLL when they visit, and arrange for a MLL meal if one is desired. Offer them (or their kids) an hour in an AC flightsim if they're interested. Send them a Christmas/birthday gift. Load a couple cans of Pepsi onto the plane if they are Pepsi (and not Coke) drinkers (or if they like 'adult' beverages, make sure their preference is on-board), provide customized meals, give away high quality sports and concert tickets through the concierges, etc.

Basically, treat those customers like royalty in whatever ways they can. Make them completely, and utterly repulsed at the thought of using the competition where those luxuries, that quality of service, won't be present.