Signs should be no larger than necessary to get the job done.  It is really important that they be laminated or enclosed in plastic if made of paper.  If attached to trees, do not use large nails that might harm the tree.  Staples are preferred and should only enter the bark, not penetrate into the flesh of the tree.  Nylon cords can be attached during the lamination process or passed through the plastic if tieing is preferred rather than stapleing or nailing.

Check back from time to time to make sure your sign has not fallen, blown, or been pulled off and causing another source of trashing.

As with anything, consideration must be given to the landowner if on private property and permission must be asked.