I used to use roofing nails.....
Then switched to the nails with the plastic washers.  They hold the sign a bit better.  The nail really needs a big head to hold the paper.  And lost of nails.  The idea of using wire fence is interesting....
My signs mainly get tore up by two things.  The sun and tree rats.  After about two years the signs on the tyvek like "paper" degrades in the sun.  It turns brittle.  I used to think someone was tearing down my signs.  Until I looked closer and you could see little claw marks all over the sign.  

  Sun light and a squirrel handing from the sign does not equal long life for the posted posts.  
You should check you state regs/laws.  In some states if you paint the trees a certain color that is considered a posted sign.  Hard to pull down paint.  
You need to check you state laws on posting.  In NC the law is very specific on what has to be done to post your land.  The sign as to be a minimum of 120 square inches and has to include a way to contact the land owner.  The signs have to be on the corners and no more than 600 feet apart.  
Alas in my state a painted tree is just a painted tree unless it is state land.  BUT I have started to paint the trees on the property lines.  Its just easier to find the line if brush grows up and it makes reposting that much easier.  I have been using orange spray paint.  This stuff is fading on the trees that get lots of sun.  

  I looked into using what the foresters use to paint trees but that stuff was high dollar.  Need to check around other suppliers for this stuff.
Later,
Dan