I'd say be more considerate and such but most of the jerk behaviour I see is in threatening pugs anyway.
when you get the more hardcore/elite/etc players, that probably use these forums too, they seem to be pretty nice in comparison, and because of their level don't mind carrying weaker & bad players. I have a better time playing in perilous pugs over threatening just because the idiots get fed up and stay away from perilous when they can't get any completions. I guess going out of your way to support fresh players will be great here, personally if we can carry as 3 I don't mind some fresh player jumping in as 4th (as long as they aren't taking the ******)
other things that cause people to leave are down to the game's empty achievement in progressing, which is game design. you can grind out promotions endlessly trying to create perfect characters but pretty much everyone will get to a point where they go "why am I doing this, it's not even fun" and leave. most of the "achievement" spurring people on is in the loot list (which is limited to a few desirable items and the rest being junk) and rankings (only top 100? expand it more, with more stats ranked so it's not just the elite timewasters who care about it)