(MONEY Magazine) – --RULE 1 Use 'em--fast. Major carriers have cut back on the number of planes flying on most of their routes, which translates to fewer available rewards seats. In effect, the value of your miles is decreasing, and the trend isn't likely to reverse itself anytime soon.

--RULE 2 Book very early--or very late. You'll have the best chance of finding an available seat if you book soon after the flight has been entered into the computer booking system, which happens up to 330 days in advance. On the other hand, if a flight hasn't sold out, airlines figure they've nothing to lose by making more seats available for rewards travel.

--RULE 3 Consider paying double. We used to say a mile was worth 2¢--so you should try to get, for example, at least $500 worth of travel for 25,000 miles. But with all the new restrictions, it may be worth spending 40,000 to 50,000 miles for unrestricted access to any unsold seat on a domestic flight. It's not cost-effective, agrees Tim Winship, editor of travel website FrequentFlier.com, but "there's a point at which you should stop doing the math and just do it because it makes you happy." Besides, 25,000 miles may not buy you a seat forever.

--RULE 4 Use miles to upgrade, especially on long flights. New York City to L.A. coach fare is running about $200 these days. Buying your way into a $1,000 business-class seat is a great use of 5,000 to 15,000 miles.

--RULE 5 Don't overpay for miles. "The minute your miles start costing you something, you've lost the equation," says travel expert Joe Brancatelli. If you're being charged $80 a year for a Visa card that earns miles and it takes you three years to earn a coach ticket, that ticket has cost you $240--enough for you to go almost anywhere in the country on a low-fare carrier.

--RULE 6 Aspire to be elite. Even as mileage programs are getting stingier for occasional fliers, they're growing more generous when it comes to heavy travelers, who are being wooed with an ever-growing array of perks. Squeak into their league; so-called elite status usually re quires 25,000 flight miles (as opposed to miles accrued on a credit card) in a calendar year--and you'll be ushered into a world of early boarding, guaranteed upgrades and reams of bonus miles. So if you take more than four long-haul trips each year, it pays to stick with a single carrier. --C.W.