Well, most any craft glue that dries clear will work, if used sparingly. Squeeze out some slow drying glue into a small cup and apply with the stick (remove the cotton) from a Q-tip or use a toothpick to apply glue to reverse of the dried flowers/foliage.

I usually lay out dried or pressed leaves or ferns to outline the underlying shape of the arrangement; then apply glue to the reverse to tack them into place on the mat or mounting paper. Let dry for a while and then arrange the dried or pressed flowers over the top until I get a pleasing arrangement-glue them down. I then spray the entire thing with a layer of clear drying craft glue; it will bring out the colors and protect everything from moisture. Be sure not to make the arrangement too heavy; it's better to leave some "air" for a natural look. You also want to be sure that you have considered the depth of the frame or shadowbox so that you don't end up crushing everything when you put it together. Let the arrangement dry overnight before putting it into the frame so that it doesn't mold.
You can use "leftovers" to embellish some all occasion cards for the newlyweds or to make smaller framed pieces for the Moms or for small pictures for "thank you's" for the wedding party.