Language is powerful, so careful consideration of word choices is in order.
"Do" vs. "Don't"
As you know, suggestions of "what not to do or feel" are generally ineffective, unless one is attempting to achieve a paradoxical effect. The mind and imagination do not filter the "don't" so be sure that suggestions are always stated in positive or "do" direction.
"Relax" Is A Loaded Term
It is important to try to avoid using the word "relax" as a command. Most likely, the patient has been repeatedly ordering her/himself to "relax" in her/his head, or has heard it repeatedly from a well-intended or frustrated loved one. Ironically, our efforts to calm ourselves can take the tone of self-recrimination, as in "Will you just relax?!?" or, as in "Get a grip!" Often, the result is increased activity in the left brain, resulting in increased physical tension. Consider using suggestions such as: release, open, turn-loose-of, soften, empty, off-load, etc.
Suggestions of Movement or Change
Imagery that is kinetic is optimal. The suggestion of movement or change – that something or some process is at work so that change occurs.
This can be reinforced by placing a hand over a point of pain or concern. Suggest that the hand is helping to transmit the change agent (energy, temperature, sentiment). The hand on the chest also helps to calm the heart or soothe lungs.
Sensations and Metaphors
The language of imagery suggests sensations and contains metaphors such as:
"Lungs designed to work like a bellows"
"Feel stress/tension flowing downstream and away"
"See competing thoughts or feelings as bubbles in the glass that rise to the surface and are released"
"Watch your heart (or muscles or blood vessels) opening like a flower as it blooms"
"See the tension melt like ice returning to water"
"Notice the calming, yet energizing effect of golden light that is all around you"