The best state of mind to come up with ideas is if you manage to feel free. No limitations and no embarrassment. The state of mind kinds can have when they just give it a try.
To help my students to trigger this freedom to create and, very important the notion that making mistakes is part of the process of learning i give them a simple task. They have to draw quick portraits of classmates, but they have to do this without looking, with closed eyes. The results have an uncontrolled freshness.