In order to create good conditions for being aware in meditation, you need to have an open chest, with a sense of spaciousness across the front of your chest between your shoulders. This allows you to breath freely and to oxygenate the blood.
You can encourage this sense of spaciousness by taking a few deep breaths and filling your upper chest.
As you breathe in, the front of your body will rise (see the arrow on the photograph). Feel the openness across the front of your upper chest and, at the same time, relax your shoulders, letting them fall and roll back (like the arrow behind this meditator).
If, while sitting in meditation, you feel any stretching across the back of your shoulders, it probably means that you need to have you hands supported higher. Some people need to have their hands supported as high as the navel in order to avoid shoulder strain.
While meditating, you may have the sensation that your shoulders are rising and falling as you breath in and out. If you tune into the sensation of your shoulders falling on the outbreath, you can encourage your shoulders to relax more deeply.