Imagine how it would sound like to play the first line of the piece you want to practice.

Now, attend to the way you are standing or sitting.
Expand and shrink in the three dimensions/panes of the room (5 seconds each):

1. Become as tall as possible, and then as short as possible - and then COMFORTABLY tall.

2. Become as wide as possible, then as narrow as possible - and then COMFORTABLY wide.

3. Become as deep as possible, and than as flat as possible - and then COMFORTABLY deep.

4. Combine all planes: become as tall/wide/deep and then short/narrow/flat as possible. - and then?? 

right - just COMFY!

While doing that: Ask questions - Do I feel pain/tension? How is my body aligned in space? What is my breathing like? How do I feel in general? How is my mental state? or else...

Imagine how it would sound like to play the first line of the piece you want to practice. Would there be a difference to before?