OK, I'm going to lift this pretty much word-for-word from my post in NC's thread a while back, because I'm at work and really should be doing some sort of teacher-y thing. Do a bit of reading on rule of thirds. It's where I usually start with scaping a tank; it gives you a lot of room for creativity, with a unified-but-natural look. Here's my take on it:
For an aesthetically pleasing scape, I use the rule of 3rds. (It's not really a rule...more like a guideline) Think in terms of focal points: Spots that draw the eye. Focal points can be hard structures (rocks or wood structures) or interesting accent plants. For your scape, a particularly interesting piece of wood, an intersection of several, or an interesting rock formation would form a great focal point.
Put a focal point about 1/3 of the way in from the side of the tank around 2/3 the height. For a bigger tank (I can't remember how big yours is), I like a primary focal point on the left, and a secondary focal point on the right, 1/3 the height of the tank.
It makes a natural-looking but coherent scape.
Something like this, with a wood structure as a primary focus and a really interesting plant (or rock, or pot, or more wood--just anything that sort of draws the eye into the scene) as a secondary focal point. These points get one's attention initially, drawing the viewer's eye into all the other interesting but more subtle plants and such waiting in the background.