Zhan Zhuang is priceless, every system has their own. I would reccommend the Zhang Zhuang methods found in Xingyi and Bagua systems.
One of the best, and most simple (yet very powerful) ones, is the tree hugging position (as I call it
). You can do it in a horse stance, or knees slightly belt, with the right posture of course (tounge on roof of mouth, straight back, wuyin tucked in etc.).Arm pits hollow, arms round, shoulders relaxed.
Another good one, is San Ti Shi, the basic Xingyi stance, make sure your alignments are correct, and stare into the space in between index and middle finger, imagine your mind expanding in front of it, but never concentrate on one thing.
Another basic one we do in bagua, is in horse stance, hold your arms out horizontally, as straight as possible. Stretch out your arms as far as possible, and hold it here. Imgine your bones, tendons, and marrow seperating while remaining calm and relaxed.
It will imrpove everything, pushing hands, jing, and posture.