One of the best perks of major and expensive cards and surprisingly some free alternatives, is the airline incidental credit. This perk is a major bonus but it needs to be used every year. So what happens come December and you've only used $25 worth of the $200 credit? Well using a fun little trick you can use your whole credit, and apply it for the future travels.
The trick is currently working as of 2018, and it can depend on other items as well, as the purchase needs to code as a travel purchase. The best way to do that is to buy an egift card for the airline. I have tested it on Delta in 50$ increments. FlyerTalk, a website for traveling professionals and aviation geeks alike, has whole threads on purchasing and using these credits. I have tested using my American Express Platinum ($550 annual fee/$200 credit) and American Express PenFed Credit Union Pathfinder ($0 annual fee/$100 credit).
Its really fun to get these right in December and email them to family as late last minute gifts. Enjoy this fun little hack and spread the word about it, except to the credit card companies.