I live in the same area but rarely fly out of COS. I'm probably a little further north of the Springs than you as it takes me about 1 hour to drive to either airport. I use The Parking Spot to park at DIA. They've got their own "frequent flier" program and especially in the winter, getting picked up and dropped off at your car is more comfortable than trudging through the peak parking lots.
98% of my flying is business. Almost all on UA, with an occasional F9 backup. I'm tall and anything over 2 hours in an RJ ruins my back, so I spend a lot of time connecting through IAH and ORD. I'll be 1K again this year, and the changes to the Mileage Plus program for next year will actually work in my favor due to the nature of the tickets I buy. I'm not UA's biggest fan, but as long as they don't convert DEN into an all RJ base, I will probably continue to keep them at the top of my list.