Since there are so many forms of moral decadence, perhaps we should wait for the problem of moral decadence to arise before defining a one-size-fits-all solution.
Arguably, the modern Western world, at least, does not face the problem of moral decadence.
Western Europe and most other Western countries are largely peaceful and tolerant societies, where social norms are generally respected. No solution needed there.
The United States may have a slightly higher serious crime rate than some other countries, but will no doubt work on solutions within its own cultural boundaries. Since America is already a relatively religious society, we can say that religion will not be the solution.
Perhaps the one apparently intractible problem of moral decadence that all Western nations seem to share is drug abuse. Prohibition and punishment of users have been tried for some decades, and can no longer be regarded as the solution. Perhaps the lead taken by the Netherlands and other countries, in treating supply as a punishable offence, but use as non-criminal, is the way to go.