Thanks for the responses. I probably should have provided a couple more details. First, most of my travel is for work, so I usually have to go with the cheapest ticket. I have some flexibility on that, but not a lot. The other thing is that the airport I fly out of (DSM) isn't very good for consolidating down to a single airline, or I would have tried to do that already. The other thing about the airport that I fly out of is that 90% of the flights I take have a connection. Finally, NWA and United are the "big airlines" that fly out of there. To be honest the Continental flights out of there are usually codeshares with NWA, but sometimes the Continental ticket is cheaper.

I can probably get 3/4 of my travel on one airline, at best. NWA is the way to go, since I didn't realize I could count Continental and Delta towards NWA.