I picked up a handy tip from a snake researcher here in Ontario, this past summer. From what I know the "standard" way to PIT tag is by going against the scales (eg: inserting towards the head). It is slightly more difficult but inserting toward the tail actually improves your chances of the tag NOT coming back out of the snake via the hole of entry. Because of course the way a snake moves the "standard" way of inserting makes it more likely that the tag will fall back out again.
This is not so much a problem with captive snakes but with wild snakes a lost PIT tag means lost data (and wasted time and effort).