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? :)
It depends on what you're going to do with your pics.......if you're stashing them, I've found that covering them with a sheet of glassine and stacking them works fine (not so good if you're moving them around). If you're displaying your pics, then I'll agree with the others and say under glass every time. Only make sure to put a spacer between the glass and the pic, and mount the pic on the backing board with some acid free tape to prevent it from touching the glass.