For leaves and plant material you can use a product that you can buy in any of the taxidermy catalogs. I can't recall the name for some reason right now, but it comes in several different colors. (I think it's called "never say dye" Green, yellow, clear, and a few more. You set the plant in it and let it draw the preservative up into the stem and throughout the leaves. Works pretty well.
I like to use dry stuff so I don't have to preserve anything. Just be careful to bug spray anything that looks "buggy" so you don't send critters into someone's house.
Dry grass, dry weathered wood, rocks, and stuff like that can be used without any preservation.
I like moss too. It can be purchased from several supply companies. It's green and brown and can be attached to almost any type habitat.
DBD