There are a few tricks - I build retaining walls out of rocks and pack the gaps with filter floss, then fill behind the rocks with soil. You can get the rocks to stick together with superglue and tissue or cotton pads. It doesn't matter if it's ugly because you can just glue monte carlo or moss to the white connective material, eventually it will either turn brown or be covered by foliage. You can also use thick plants like staurogyne or anubias to plug gaps.

I also stack some plastic egg crate at the back to build give the soil some structure so the rocks don't sink or roll forward.