You have to add more miles every 6 months.

Fly Frontier.

Get the credit card and use it from time to time.

Rent a car and pay the fee for Frontier miles.

Transfer points from Marriott.

Earn points by staying at a Wyndham Hilton or LaQuinta

Transfer point through points.com

Buy some Flowers through teleflora

Signup for e-miles and/or e-rewards

I have found that Thanks Again! which covers most credit/debit card purchases at some airports (ATL in my case). It only earns a few miles but those are in addition to whatever miles/points you earn otherwise. You can transfer as few as 100 points to Frontier to keep your miles alive. Thanks Again Points also expire but if you earn a few points with them ever few months they get kept alive. Thai one requires some management.