You might consider flying Frontier more frequently. Their fares are usually considerably cheaper than anyone else. If you buy Classsic Plus tickets you get 150% of the mileage flown, 125% for Classic tickets. A fare from Phoenix to Denver on Southwest is 299.00 plus tax for Business Select and 113.00 plus tax for Classic Plus on Frontier. On Frontier you get 5 extra inches of legroom, 2 free bags, free TV, no change fee, and the fare is refundable, plus you get the extra miles. And, you can redeem a free ticket on Frontier for fewer miles than almost any other airline.