Easiest way to prevent miles from expire

Almost all North America Airlines miles have expiration date(normally 18 months), but any activity can extend the expiration date for another 18 months. Online shopping mall is an excellent option, the cheapest merchant is Itune, all you need is 99c to buy a song and extend your miles. Itune is also a partner of Hilton Shopping mall, so you can use to extend your Hilton points for another 12 months as well.

Most Asia programs have fixed expiration date, you can not extend them, either use them or lose them.

My favorite BMI program has no expiration date.