Before I put my 2 cents worth in, let me say that I do not consider myself an "aquascaper" in the artistic sense. I just try to put something together that resembles a natural environment and is pleasing to my eye.
I put the hardscape together first, since I can change it many times without water in the tank until I'm satisfied with the general layout. The entire time, I'm trying to imagine the plants softening the lines and the hardscape highlighting the plants and breaking up the green monotony (low tech, I can't keep reds alive for long).
I typically have an idea of what plants I'm going to use. Pretty easy considering the short list of plants that I have been able to keep alive and thriving.