Tough one. Long term, a concerted effort by the outdoors “industry”, government, Sierra Club, influencers, schools and perhaps the DMV (or other group that comes into contact with just about everyone) is needed. In general people need to become more aware of caring for the planet.  Public service announcements would be helpful.  Short term, probably needs to be kept simple.  “Leave no trace” always resonated well with me.  Perhaps a simple sign post stating “Please - Leave No Trace” would help.  Beyond that associating the “leave no trace” notion with a symbol (like the no smoking symbol - except for leave no trace) would work.  Then this symbol could be posted in the same way trail markers/blazes are posted. 

“Take care of the earth”