Use a golf ball, stick and your own hands to massage your feet and release tension you didn't realise was there.
In Thailand they love to massage their feet. Half the bones in the body are in your hands and feet. If you wear shoes a lot then your feet are in a little box and can't move so much and they miss out on free massage that comes walking on the ground.
I've kept this one pretty simple, but it's enough to be effective — just try it, even a minute a day standing on a golf ball and you should feel the benefits after a couple of weeks. It can be sore at first, but fairly quickly it becomes easier. Partly your mind is getting used to the sensations and accepting them more easily. More importantly, dozens of tiny muscles in your foot are learning to let go of unnecessary tension, which can in turn help your leg and posture to softer and more relaxed.