Here is my suggestion -- start looking at planted tanks (people who keep live plants with their fish) and find out what you like. You can start by looking at the Member's Aquarium and Fish Pictures board on TFF or you can just look up "planted tanks" on Google. After looking for a while, you'll certainly get an idea of what you, personally, find aesthetically pleasing...
The other thing that looking at planted tanks will do is that it will give you an idea of the way plants will naturally grow when left alone in a tank. Does this plant stay spindly and grow taller? Will it branch and become very full? Will it spread from the root system and start to fill a corner of the tank? Knowing these things will help you to replicate that look in your own tank. Once you know what you want your tank to look like, you can try to re-create that look in your own aquarium with silk or plastic plants. Making a "fake" plant look real may require you to bunch two or three silk or plastic plants (of the same type) together, but you will be much happier with the result.
Finally, I will say that most aquatic plants (live) are considered to be either foreground, midground, or background plants. This has a lot to do with how tall (and sometimes how full) they will be. Following these guidelines will help you to have a more pleasing tank as well -- taller plants in the back/center, shorter along the front/sides. Also, there are "specimen" plants -- plants that are used as a feature piece, perhaps in the center of the tank, because they are particularly large, attractive, or just plain cool. The Madagascar Lace Plant is a good example of this. You might want to try that as well.
Just some thoughts about "planting" without live plants. I really do think you can have it all nowadays -- the silk and plastic plants can look surprisingly real, and require far less care than most live plants.
