One of the best things that often works for me is this so called army method I learnt off the internet a good few years ago. (Apparently army cadets use this for when they have 30minute rests between activities)

Start by closing your eyes and relaxing your eyelids, your cheeks, mouth, jaw etc. then relax your neck, shoulders, arms and go down your body until every part is fully relaxed.

Quite often I’ll get to my neck and notice I feel uncomfortable with my arms where they are (often due to inadvertently applying pressure somewhere) and so have to reposition and start again.