It is said that Ms Maathai, Assistant Environment Minister of Kenya, was impressed by the Japanese word ‘mottainai’. The idea of ‘mottainai’ is fairly well expressed by a Japanese college student as: It expresses not only regret for the material loss, but also regret for the process of making, getting or achieving something. This is the "mottainai" spirit.
I have gathered several translations from newspapers, etc.
1. Waste Not, Want Not.
2. What a Waste!
3. Don’t Waste Resources.
5. Too Precious to Waste
6. Don't waste what is valuable.
7. Don’t waste it because it’s precious and limited.
8. Shameful to waste
In my opinion, these all work and mean nearly the same thing. As a slogan, which one, do you think, is the best? ‘Mottainai’ is basically an adjective, but as a slogan, it need not be an adjective in English. Also, do you have any other suggestions?
4. Never throw away easily.
4. Never throw away without thought (or, without thinking.)
A phrase we use in my city is "Use Less Stuff." ("Stuff" means things, objects, items) "Use Less Stuff" is simple, short and easy to remember... we also hear "Use less" Very short, but it means exactly what you want to say.