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 large accent plants. For your scape, a particularly interesting piece of wood, or an intersection of several, 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 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.
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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.
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