Stretching has never worked for me either. I always end up having to trim them. I purchased the best pair of scissors I could find at Staples and VERY carefully trim the seal until it is slightly tight, as opposed to slow-death-by-strangulation tight. (They do loosen up slightly over time)
I put my special sissors in a secret place and I don't use them to cut other stuff, and I never run with them.

: I tried stretching mine but could not get them anywhere near
: comfortable. I ended up having to trim them. If you are very
: careful and use a new razor blade for each cut you can trim off
: one ring at a time until you get a good fit. I found that if I
: stretched the neck slightly over a mixing bowl and made one
: smooth even cut around the gasket it was pretty easy. you just
: have to be careful to go all the way thru and not leave a nick
: or ragged edge and make sure the start and finish of the cut
: line up or the gasket could rip.