A big piece of wood and some plants! You can go to a creek or river and search for some cool dw. Just boil it or bake it, let it dry and throw it in! Here's a piece I found awhile back:
A big peice of wood with plants growin from it. This is mine mopani wood and fake plants. Ya i dont love the bright purple and orange but little cousin wanted it, made me put them in right when we got home from lfs
I'd say to start out with a background, it doesn't have to be a 3-D background (although it'd look nice), just a normal black back will do. That will clean up the back of the tank by hiding your wiring and disguising your heater a bit.
I'd add at least 1 piece of driftwood, Manzanita is great and I am also a fan of Malaysian.
Fake plants probably wouldn't look too bad either. Take your time and pick some out that you wouldn't mind seeing everyday; good looking fake plants are hard to find but IMO look pretty good in a well designed tank.
Last but not least, go around the site and check out lots of different setups; tank of the month is a good place to start. That way you can get an idea on what you want to accomplish when re-scaping your tank.
Good luck changing the setup, keep us updated with pics!