If you use a styrofoam base (and there are some nice ones, different shapes now in stores)..you have to use special glue for styrofoam (this is like extra thick Elmer's) It is not quick to set, so you have to hold it, prop it, or something for at least 15-30 minutes (the heavier the longer)

For the slabs on wood I use E6000 Clear. (not the epoxy type)
http://www.eclecticproducts.com/e6000.htm

If you want an exciting thing...there is a coating resin, which I am investigating myself, called Envirotex Lite.
This will shine up and make rocks look spectacular.
They use this in bars and restaurants to coat table tops... but the rock guy I know coats his large pieces with it...
http://www.sculpt.com/technotes/Envirotex_tech.htm
It is spectacular--- really! So I am planning on getting this soon. Maybe will take on vacation, since temp is important, and it is too cold here now even indoors for it.
You could glue down your leaves...to any clean surface and pour the Envirotex over it and make a "painting" or plate or whatever... Believe me the final result is fab..and it does not yellow.

Here is another site with ideas...
http://www.save-on-crafts.com/enpr.html
you can do an infinite number of ideas with this stuff!