It has become increasingly difficult to place a monetary value on frequent flier miles. At one time, budget travelers operated under assumptions that each mile was worth perhaps one U.S. cent, so 25,000 miles would be worth about $250.

Such assumptions might not always tell the whole story these days, because frequent flier miles are becoming more difficult to use. In addition, many airlines now charge their customers for the privilege of redeeming these miles.

So much for rewarding brand loyalty. With all of this in mind, you must decide if a sale fare could be a better deal than redeeming your miles. Don't cash them in if a low fare is available.

But above all, don't waste your miles. The worst mistake of all is to let them expire.

More information: Avoid Mismanaging Frequent Flier Miles