Sometimes I want leaves to stay "fresh" and not turn brown. This is how I've done it successfully in the past and how I did it today. It's very easy, quick, and preserves your leaves for years.
You will need:
Gel medium (gloss or matte, your choice)
foam or other brush
Begin by spreading a small amount of gel medium on one side of the leaf using your favorite brush. When dry, turn the leaves over and brush the other side. If you plan to leave the stems on the leaves, be sure to brush them, too. When both sides are dry, use gel medium or your favorite glue to add them to your paper project.