Short ones:
Refuse Plastic, be fantastic
Use Cloth, Dump plastic
Go environment friendly, be plastic's enemy
Say no to plastic!
Bring along a basket, do not use plastic
Plastic bag is a death bag
Be a super saver switch to paper
Reduce Reuse Recycle
The smoothest destroyer- Plastic
Plastic is drastic.

long ones:
1.Don't make your planet a deathbed of plastics.You owe it more than this! 
2.It takes more than a millennium for a small piece of plastic to disintegrate.Should the future generations suffer because of our mistakes? 
3.Make your planet a better place to live by using paper bags instead of plastics 
4.Don't suffocate yourself with plastics. It may kill you
5.Stop bagging the planet -- say NO to plastic bags." 
6.Plastic Bags Blow
7. Make world green not plastic
8.Don't be a plastic junky
9.Do you want a green planet or a plastic planet???
10. Follow the three Rs--recycle, reuse, reduce