OK, then the first thing to do is to make sure that the reels are ABSOLUTELY dry.
It's worth practising with the reels until you really know by feel which bit is which, too.
Like Fen, I snip the sharp points off the corners of the film - stops it snagging.
Then I apply a very slight curve across the film when inserting it into the reel, and pull it as far as possible onto the reel before winding it with the customary back/forth motion - which has to be done fairly carefully with 120. Keeping the backing paper away isn't always easy - you have to find a method that works for you, but really, it's a case of being aware. And also like Fen, I find it easier with my eyes shut!