always, this is due to nervousness or discomfort with being touched, or 
with being unclothed.  (See question 2.4 about sexual massage.)  This can
be dealt with by massaging first less threatening areas.  The back is
perhaps the least threatening area to massage, followed by (in most cases)
the limbs, neck and head (including the face).  By the time you reach more
sensitive areas, i.e. legs, chest and abdomen, they will have relaxed from
the effects of the massage.  They will also have had a chance to develop a
level of comfort with you that they feel safe.  Note this assumes that your
attitude and approach to massage are consistent with such feelings.
Increased pressure will also reduce ticklishness, particularly if their
sensitivity is physiological rather than emotional.
        lonnie@meaddata.com recommended: The easiest solution is to have
the person receiving the backrub place their hands on or near the hands of
the person giving the backrub...  After several times of using this
technique the recipient will no longer need to touch the masseuse.