Upgrade with miles, not dollars.
Credit: Chris McGrath / Getty Images

A recent round-trip flight from New York to San Francisco on Delta cost $699 in economy and $4,117 in business class – that's six times more expensive. A cheaper way to do it: Buy the economy ticket and upgrade for a measly 25,000 miles (and maybe a small fee). The same number covers a coach ticket on that flight, so upgrading using miles amounts to paying just twice the price.