When Channel 4 first started it ran " The Body Programme " with Yvonne Ocampo. It was there that I first came across the term " Ideokinetic Facilitation "., which was about using mental imagery to bring about postural change.
Tai Chi is full of imagery - root, iron bars wrapped in cotton wool, embracing the tree and so forth. Working with imagery achieves better results than trying to force the body into a position. An example might be to imagine that when standing straight you are in the middle of two slices of bread, like a sandwich. The front slice is sliding upwards whilst the rear slice is sliding downwards. This has the effect of stopping you from slumping and promoting a more upright posture.
A really useful book is A Tai Chi Imagery Workbook by Martin Mellish, a Singing Dragon publication. It's full of good exercises. Imagination becomes Reality?