I'm not an expert in aqua-scaping but just to share a few simple tips that can make your tank look better.
Firstly, you must use sand instead of other bigger substrate like gravel.
Sand will make big impact on the overall look.
Look for sand that is lighter in color and not too dark. Dark color sand like black will make your tank look dirty.
Lighter color sand will make your tank really look like a "neat & clean" river/stream.
If you are good in matching of colors, you will understand this point better.
I had tried different color sand and gravel and nothing can beat a light color sand unless the person has bad taste of colors.
Secondly, get 1-2 big driftwoods.
Smaller driftwoods can be more difficult to arrange them.
Big driftwood that can cover 1/3 or 2/3 of the tank will give bigger effect on your tank.
Look for driftwoods that have more branches like a tree.
You can place them like trees growing upward or you can place them upside down to make them look like roots inside or above the water.
You can add some rocks - big and small mix.
Use either a black or dark blue background. Other colours will be hard to match.
Here are some basics on aquascaping.
