Over time I have noticed a way in which a scene that is busy visually can still retain a sense of sonic cleanliness, fluidity and impact. Of course, 'less is more' - keeping the number of layers used to a minimum is a valuable approach for a well-designed sound, but when there is so much to take in visually how can you keep faithful to that adage?
Let's take for example a war scene. It's so easy to get excited with how much is going on visually that the sound effects edit can turn into a block of indiscernible chaos. I have found that often the best sounding scenes that are visually complex are the most simplistic sonically, which demonstrates the 'less is more' saying perfectly. However simplicity in design is a very difficult skill to acquire.
If you are having trouble with a scene, perhaps approach the FX lay in this way: What is the focus of the shot at the current time? What grabs your eye on the visuals first? Lay ONLY that sound. Do this for every shot in the busy scene and begin to build up a rhythm sonically to gain the fluidity to carry the listener through the scene. Strip the shot to its core. Why has this shot been added? The accompanying sound should reflect this.