Shoe Goo just paint a few layers on. works fine. also does a good job of glueing rubber on if you feel you need to but i have found a couple of layers of it works fine, good friction and good protection of your toes.
Stick 'em in the oven, they'll come apart then. gas mark 10 2 or 3 should do the trick.
I tried to soften an over-tight pair of new boots that way a few years ago. Hadn't bargained on the glue being heat-sensitive.