With the new devices to prohibit such large pieces onto the plane, customers still seem to wanna pack it in. If your bag is too big, check it, and wait for it at your destination. Yes, I know that can sometimes be aggravating, but at least you won't upset everyone else on your flight when you are holding up take off because you cannot find a large enough overhead bin. Avoid the middle seat if at all possible. Plead with the person at the check-in counter to have you seat changed.
Airplanes are tight enough why experience more pain by cramming between two other passengers.
Make reservations early you get better selection and price. Two weeks is the standard. It means something. Yes, maybe you get a better fare from Denver to Los Angeles.