I have had to do this many times.
While I like an intricate detailed scape,some of the larger species make this impractical.
I have found more modestly furnished tanks can look good if you have an eye for placement of key decor items.
Things must have a good composition and the rule of threes works well in achieving this.
I also like to use a black background. Not only does it show the fishes colours off well but it creates a feeling of depth.
I also like to decorate the tank from the top down.What I mean by that is to keep the substrate relatively clear with a few large carefully placed rocks.The rest of the furnishings are branches attached to the rim of the tank or the tanks brace bars.
These will hang down from above and provide a furnished look while keeping the bottom half quite open for larger fish.
Another important aspect to take into consideration is scale. For big fish use big decor or things just don't look right (at least to me anyway).
The substrate should be deeper at the back of the tank and slope down thinner towards the front.This also helps to create an illusion of depth.
One more thing to consider is lighting.
I prefer a more spotlighted look with darker corners.This will add interest and also create the illusion of depth as your fish dissappear from a bright area to darker corners of the tank.
It can be difficult getting it right,I have been having this problem with my ray tank.
With the right composition and lighting,less can sometimes be more and the bigger your show fish gets the less influence on the overall look your decor will have.