Not sure there's a silver bullet for this one, but you'd have to think yoga and green tea can't hurt unless taken as an enema mid pose.
fwiw my hip flexors are TAF from sitting at a desk all day, and I notice a lot of things improve when I stretch them, including comfort on the bike, and sleeping too. I'm also the opposite to advice given here, if I have a more stretched out ride it results in more acute angles around the hip & torso, which sets off those muscles and shortly after my lower back as well. Boo.
PS I did get an mri which indicated disc degeneration in the lower back but all normal levels apparently, as the possible cause. Nothing I can do about that AFAIK.