Rules for Being Friends With Me.

1.  Feel free to talk about me behind my back at any time and with any one of your choosing.  You can even talk about me when I am present.  The only caveat is that it must all be true and can be proven.  When you start to spread lies and rumors, our friendship is over.  Lies that are malicious and harmful to my reputation are special deal breakers.  Lies that are told to cover up your own inadequacies are just mean spirited and rude.  Accept your own shortcomings and don't pawn them off on someone else.  (That really should be rule #2, but I will leave it here.)
Perhaps when you are talking about me behind my back (or even to my face), you can mention some nice things I have done for you.  You know those things.  The ones that I did because I thought we were friends and wanted you to be successful.  Or just because.
When we are no longer friends, because you have broken Rule #1 - you can say whatever you want about me.  Lies included.  Because I believe that someday it all comes out in the wash.  People who choose to believe the lies will someday have their eyes opened to the truth.  I know, because it happened to me.