If you're trying to achieve MVP Gold, don't worry to much about the miles as you can also achieve this level based strictly on segments.
Advise by msimpson on cheap "mileage runs" is good also. I just got back from one last week on NW. Did PDX-SEA-DTW-FLL-MEM-MSP-PDX for $228 total. By flying the redeye on SEA-DTW, I did an immediate turnaround in FLL (landed around noon, started heading back at 2:00pm) and thus didn't have to pay for a hotel room. Take care.
CS