Marcie - miles (on most of the airlines) expire in 18 months only if there is absolutely no activity on the account - either flights or other credits (such as credit card / dining / other partners etc) or redemptions. Once there is activity on the account - the calendar "resets". So credit card / dining will extend the "life".

Or just make a habit of dumping a tiny bit (I believe the minimum is 1,000) of your Starwood points or American Express Membership Rewards points into each of your frequent flyer mileage programs on a preset date yearly - eg Jan 1.