You can't prevent it entirely. It's the nature of plastic to rot over time, if my memory from plastics class in college serves me correctly...been awhile.
The carbon molecules in the plastic start to "cross link" over time, which causes the material to become brittle and "rot."
As others have said, best thing to increase longevity is store your Gs out from direct sunlight for long periods of time when not wearing them as UV rays cause cross linking in plastics. And do your best to not expose them to chemicals and deet insect repellent.
You will eventually have to replace your strap and case at some point in the future. It can't be avoided. Luckily though, swapping the outer resin case and straps on Gs is easy prays.