"Is there a trick to getting a good rate on a one-way rental? I want to go from DEN to PHX in August over an 11-day period and I need something with six seats. I've tried various BCD codes and the best I am finding is about $1600."

Arizona and Florida often have cheap one way rentals to get cars to and from there to coincide with snowbirds and tourism; meaning cheap one-way fares to get cars OUT of there in Summer and cheap one way fares TO there in Fall. They may not be posted this far in advance however, so probably not much help at this point.
Maybe you could just get a reservation on hold and then check back closer to Aug. Not sure on the exact schedule, but August may also be a little early for Phoenix to "car up" for the season.

$1600 seems a bit steep. Maybe other options besides car rentals?, like people needing a vehicle driven?