I would first say to read these threads in their entirety, as there is a great deal of insight and information on how to create a beautiful aquascape.
Then, I would examine this page:
There are some amazing tanks on that page, I picked the IAPLC winners page because it shows a lot of good examples of many different styles, some similar.
They haven't added the 2012 winner (Amazon) yet which also deserves recognition.
1. A clear theme in mind.
2. Hardscape selection based on theme
3. Arrangement of aquascape
4. Planting elaborately and very detailed.
5. Knowing your plants. You need to know how plants grow and what plants work well together.
Those are the simplest of terms I have gleaned from studying layouts, reading Aquajournal, listening to good scapers, watching them work (ADA View with Amano scaping is interesting to watch and alot of things you can pick up) and comparing it all.
You mentioned sand and gravel, that is because it adds a level of complexity to the layout and gives it a natural feel. The more elaborate the layout the better it will look, attention to detail is key. When you look at nature, it is so complex yet simple at the same time. That is what a layout needs to achieve to be atheistically pleasing.