In Never Market Again I mention how

Many practitioners focus too much on the negatives.  They talk about it [needles] like it’s going to hurt, but not much.  This leads patients to expect pain – and they’ll usually prove themselves right.  (The placebo effect is based on the same kind of self-fulfilling prophecy.)

I also spent a page or two talking about how important it is to set patient expectations. 

Chuck McKay over at AdvanceYourPractice.com explains more.  Think about it - dentists have the same problems as acupuncturists with fear of needles!  He makes some good points about how setting expectations can help with referrals.