I like to use glues that stay flexible: often called "shoe glues". modern ones work wonder.
The old and trusty neoprene or cyanoalte work also.
But the most important part is to cut the edges of your pad at an angle so that water or your body do not tend to catch the edge and lift it:
See:
www.gong-galaxy.com/forum/viewtopic.php?f=32&t=1995&p=46247
Also, rolling a bottle on the pad edges to be sure the glue sticks well, plus letting it cure 24h in warm temperatures helps a lot. I have even made myself a special "roller" to apply pads...

See:
www.gongsup.com/forum/viewtopic.php?p=29295#p29295