things that helped my hips loosen up
1. less plow pose - used to do it constantly for grappling and i got so used to folding that i did it at times when i shouldnt' have. i can actually do the plow better now that i've toned it down. maybe the better balance between muscle groups helps form or something
2. swings would prob help, but make sure you go up and not back at the top of the swing. some people emphasize the hip pop so much that they lean way far back when they swing the bell up (basically hyperextending). the swing should mimic jumping out of a box squat. if you rear your torso back when you jump from a box squat, you'll flop right back on the box in a ridiculous looking manner.
3. make sure to forcefully throw the bell back down between your legs. this will cause you to tighten your abs properly during the swing. have a friend throw the KB down at the top of each rep. the swing will get really hard really fast. after like 10 swings, have the friend stop. then try to mimic the same feeling by pushing the bell down really hard at the top of each swing. result - you tighten your abs so that you don't hyperextend your spine and your hip flexors must then stretch instead. TADA. this act will cure most a lot of problems with people's swings. when i show guys how to first do the swing, i go through and make sure they're doing everything safely and give the fundamental stuff (wall squat and towel swing and all that junk). normally after all that, they'll get that novice kinda swing. pushing on the bell for them makes their form look exponentially better in a single set.