As part of the Focused Pilot safety program, USHPA developed Flying Site Awareness signs that can be used to delineate an area where flying activities occur. Just take the PDF to your local sign-printing shop and then post it! Note that there are also online sign-printing shops, and many office supply stores can do sign-printing.
Biwingual sign (PDF)
Hang gliding sign (PDF)
Paragliding sign (PDF)