After finishing a dish of food the plate should always be licked clean, before being rinsed (optional). This offers the following benefits.
Wastage is reduced, and awareness of the importance of avoiding food waste is raised.
Mechanical licking action and salivary enzymes lower the need for polluting
detergents (washing-up liquid).
Organic loads at waste-water treatment works are reduced. Less nutrients are discharged into rivers and coastal waters.
Extending the meal in this relatively low-calorie fashion allows more time for physiological satiation signals to kick in. This should reduce obesity rates.
It is hoped that this will become a societal norm, and that omitting to lick the plate will be seen a major social faux pas. While more a meme than an invention, some markets for fashion accessories may develop, such as flexible (and lickable) nose-guards for the nasally well-endowed.