In addition to what juicyjuicee said, only leaveit on for a little while at first. Get him used to wearing itfor longer periods of time. Make sure that you keep an eye onhim and take it off if he looks scared.
We tried two types of harness with my little Mocha. She hatedthe first and tried to rip it off and wriggle out of it, even got herleg stuck once. The second one she went into a panic as soonas she was on the floor. She was racing around the apartmentterrified until she got the harness caught on the underside of her cagedoor and started thrashing. We were sure we had killed her,but thankfully she came through with no injuries. Needless tosay, I never tried again.