Book your itinerary as a set of two one-way flights, rather than as a round trip. Every time I have looked, a single one-way is more expensive than a round-trip between the same locations. Maybe I've just been 'lucky'. Edit: no, I'm just ignorant. Turns out domestic one-ways are more proportionally priced versus round-trips.
To wit: when switching in Kayak or Hipmunk from roundtrip to one-way and vice versa, the fares roughly or exactly match what you'd expect from decomposing/recomposing the parts.
This was hardly ever the case in the past when using the airlines own portals, or Web 1.0 aggregators.