Here's what I do -
Have you ever listened to a riff that ignited your imagination? You know, you hear something, then you can imagine where it should go next? That's what I do. If I come up with something I dig, it will usually trigger things in my imagination, then I start hearing things in layers, if that makes sense. Literally, when I hear something I like, I will hear 5 or more layers of instrumentation on top of it and I just try my best to move what I hear in my mind through my fingers lol. It doesn't always work, but once you get that one riff down, it will give way to the next part. If you don't have it naturally, you will develop the ability to think in layers with your music if you keep at it.
HTH!
