If it is oil pastel, you can add a varnish to it. If it is dry pastel, however, do NOT. Don't even breathe on it! lol It needs to be behind glass.
To mount it on posterboard, carefully turn it over and put rolled masking tape on the back of the pastelboard, not the posterboard. Then gently turn it over and eyeball where it should go on the posterboard. Place it on there gently.
I juts discovered that you never spray fix dry pastels. A varnish dulls the colors. And Lori is right...it goes under glass and sealed tight! Achoo.......oh oh....just lost my drawing? :)