Some things that help really loosen up my hips for sumo are just more sumo work. My stance is semi-sumo, but if I feel too tight from not doing enough work or just sitting down too damn much, etc, I'll throw in more higher rep technique work with some different lifts, like wider stance deficit pulls with an emphasis on hip torque and tension, as well as wide, low box squats.
Those exercises are pretty good warmups, too. The wide, low box squats are simplest and my personal favorite for prepping my hips.
Also, watch Dan Green(e?) deadlift, especially how he tightens up and focuses so much on position.