Generally you should do as little as possible during an acupuncture treatment. Some people recommend breathing into areas of tension that may ebb and flow during the treatment or other techniques, but I feel it is best to just do nothing. For myself and the vast majority of my patients they simply fall asleep - so there isn't much to contemplate. This will depend, however, on the points used and the environment, and, I suppose, the practitioner - but acupuncture is rarely anything other than relaxing.
What you don't want to do during a treatment is over control your breath by using some sort of breathing technique or anything else that will cause you to over focus on the different sensations that come up. This is because the energy in your body will flow in its own natural way and you just want to let that process happen to whatever degree you can. The breath, visualizations, etc. can stop this process from happening naturally and will alter the effects of the treatment.
The same is true for Tai Chi and Qi Gong. In the beginning you simply want to focus on learning the moves. After you have them memorized you will find your mind simply doing nothing while doing the form. After a year or so of practice your breath will naturally coordinate with the movements with very, very little your control being required (in reality none). This is particularly true if you are doing the moves correctly, so good guidance is crucial with Tai Chi. After a few years and with proper instruction you can accentuate some of the movements with the breath, but you need to be careful with these techniques as it is easy to do them wrong and cause more harm than good.
Over control of the breath in the martial arts, yoga, etc. can create a problem called "running fire" in Chinese Medicine. This condition is a disruption of the natural flow of the meridians by over controlling them with your breath, mind, and/or incorrect movements. I've discussed this in a few previous posts ( like this one ).
So the golden rule, in my opinion, is the less you do the more that will happen.