I fly all my short flights on Southwest, and use the freebies for longer domestic flights. Southwest Rapid Rewards earn at an accelerated rate, and earnings for the ancillary expenses - rental cars and hotels - typically feed that account faster than they would others.

I fly United for all my longer hauls, and use Mileage Plus freebies for international business class for me, and sometimes for my son and me. I use the other few airlines' freebies for transcons during the very off-peak times. I mainly mean DL, which has just about closed out opportunities for redeeming awards. Wish I'd used them up before.