I just finished doing an install on my new 7 tap kegerator and thought I would share the method I found worked super easy.
Get a long punch with a very slight taper. I suggest getting a new one so it does not have any imperfections from pevious use that will damage the barrier on the inside of the tubing. Heat the punch not the tubing with a heat gun. It does not need to be all that hot. Once the punch is hot insert it into the tubing it will expand very easy. Pull out the punch and insert 1/4 barbed fitting immediately. It will shrink as it cools and it happens quickly. You end up with a fully inserted barbed fitting without any distortion to the tubing.