Have you heard of airfare envy? It’s when you buy a ticket and find the same itinerary for less money afterward. The good news is that there are a handful of airlines that will give you a voucher for the difference if you ask. They are Alaska, Southwest, United, USAir and JetBlue. So how do you know if the fare goes down on your particular flight? There is now a site that e-mails you when your flight goes down in price. It's called yapta.com and you must register to get the alerts. Then, you must jump on the deal immediately. Of course, it will take a little while to figure out if it’s worth it. But for now, it’s a great way to save money on flights. On the other hand, keep in mind that some airlines charge outrageous fees if a cheaper flight comes along and you want to buy it. American and Continental charge $100. Delta charges $50, Frontier charges $35 and Northwest charges $25. Shame on those airlines, says Clark.