I've been slowly refining my riparium set up, nothing formal, just letting the bits and pieces that come into my hands find their way into the tank.
Waiting on some more wood that's coming in this week, but I'm finally liking the combination of rounded river rock and manzanita branches. Substrate is fluorite with a scattering of light colored polished aquarium gravel on one side; the smaller rounded gravel softens the affect of the larger rocks.
But what's really pulled it together is the plant selection that's slowly evolved. This tank gets whatever is left over/not working from my others--and the plants I have now are strongly reminiscent of some of the forest clearing marshes I remember from back east. Main plants are pennywort and a native variant with the same light colored rounded leaves that echo the shapes and lighter colors of the river rock/pebbles; then there's mermaid weed--high contrast with heavily serrated edges. Some swords, a bit of java fern, some hygro bold and a few others bits provide some texture but their dark colors don't stand out and the shapes read as realistically "weedy" without being too messy. Letting the tannins stain for a moderate blackwater affect helps too.
I've another tank with one eye catching piece of mopani set up at a strong angle in the tank--around it and under it to prop it up are jagged chunks of a pinkish granite. Off to one side is a thin slab of the same granite that serves as my shrimp feeding platform. Reminds me a bit of a creek bank with rocks tumbling in over a caught branch.
There's no magic rule; just a matter of developing your eye for design.