I got this idea from my sister, who has a mouse cage with decoration behind it, tried it with my tank and it came out pretty good.
I happen to have space behind my tank on the stand it's sitting on, the stand is attached to the wall. Anyway, I bought some styrofoam "fake-plant-holder-block-thingys" at Wal-Mart and some assorted silk plants. I made a couple of small arrangements and put them behind my tank, along with some lava rocks I had from an old freshwater tank.
The effect is that when viewed through the tank, It looks like it's part of the tank, giving it more depth. I'm sure some of you could be a little more creative, but I just wanted to share this.
I was going to add an acrylic box with the decorations in it, but this worked out fine for me with my setup and saved the expense of the acrylic box.