Good links... I'll have to try some of these out.
There are three things that Zen traditions typically use that might be of use to someone:
1. Makkoho exercises
. This is the traditional "pre-meditation" stretching routine often recommended for Zen meditation. Just Google images the word.
2. One thing that I have very really
helpful in aligning your back
is to do something that my old Zen teacher taught me. Once your are seated on your cushion, fold your entire torso over towards the floor, like a plank. You can rest there for a few seconds to make your thighs loosen up. Once your head is on the floor or as close to the floor as it's going to come, put your hands and the floor and arch your back backwards (like you wanted look at the ceiling, or like the Cobra posture in yoga) without over-stretching. Once your spine feels a little looser, come back up and do the next exercise below or start meditating.
2. To get squarely centered on your cushion
once you are already side and have taken care of your legs, start swinging your entire body from your lower back to your to your head in very wide arcs from left to right, like a metronome. But keep your buttocks in place. Over the course of 10-30 seconds, you make those arcs increasingly smaller until you find the center. You can also do it front to back.
There are videos of this movement all over the internet, but for some reason I can't find one now. Because of that, the next best thing is a weeble wobble advertisement from the 70's in the US.
Check out the weeble wobble at 0:17-0:18
There you go! Just a few ideas from a different tradition to give you some options