Southwest offers free changes... even for the same flight. I got a Sat 4-30 flight from MKE to LAS for $128, flight from LAS to RNO on 5-3 for $75, from RNO to LAS on 5-5 for $75, and a flight from LAS to MKE on 5-6 for $119.
I checked every day and booked when I thought the fare was acceptable. Then I kept checking and grabbed every single price-down that I could find.
Even after you book on Southwest, check fares every single day. Fares are lower during mid-week.