I fly frontier every now and again because I live in Denver, and I can do short notice trips to vegas for under $40 round trip.
And if for some reason, I do not go to vegas within 6 months, I just transfer 100 kickback points into 46 frontier miles on points.com and it resets my expiration.
Most of the local conoco stations give kickback points for gas purchases. Actually, a lot of various gas station loyalty programs can be transferred to frontier, so worth looking into if you have tens of thousands of miles about to expire.