Three ideas that stuck with me for a while: 

1. Ms. Scholl, not my second grade teacher, but the one in the room next to mine:  "The world is full of entirely too much packaging."  I was in second grade in 1989, but this idea has stuck with me since then, and has gained greater significance just recently.  If you stop and think about it for a second, everything comes in packaging.  And that packaging comes in packaging, which is further packaged, and so on.  Heck, there are people employed full-time in the art of packaging engineering.  And where does all this packaging go?  Landfills, where it will sit for eternity because almost none of it is even remotely biodegradable.  I was recently in the habit of eating frozen meals for lunch, which consist of a plastic tray, covered by a plastic film, all in a cardboard box.  The cleaning guy at work came to empty my trash one day and made a comment about how I produced the most garbage out of all the people in my office area.  I wasn't too concerned at the moment, but the more I thought about it (for the two years since), the more I notice it and am disturbed by it.
2. My dad:  "A car is the worst investment you'll ever make."  It's true, but there really isn't an option where I live.  Every other major purchase you make will either go up in value or stay the same price.  Only with cars do we dump tons of money into monthly payments, then tons more money on maintenance and repairs, only to be left with next to zero return on investment.  The payoff for investing in a car is the privilege of sitting in traffic.
3. Mrs. O:  "Leave it as it was."  This was in reference to nature, especially when we humans enter the wilderness for hiking and camping visits.  Don't break any branches or rip out any plants.  And especially don't add anything that wasn't already there (i.e. garbage).  Leave it as it was.  I still follow this advice 15 years later when I'm hiking.