There's no rules.
If I am stumped on something like that I leave it and usually, out of the blue, something will pop into my head in a few days or maybe years, and that's ok.
Here's the thing: when I do my best work, it feels like I am a channel for the other side, all the spirits that have ever lived. I can't effect them, all I can do is be a channel, and when I do my best work I feel I have little to do with it. It's like something comes "through" me.
It's very much like having a dream. And what you are asking is similar to "I want to dream that I'm in France tonight when I fall asleep, how do I do that?".
So the best thing for me is to not try and let it come.