James Gurney's wonderful website shares 7 ways to activate your imagination. The list provides explanations of the methods. Here's a summary:
1. Imagine a story behind the model's pose.
2. Keep a sketchbook dedicated solely to image generation rather than observation.
3. Refrain from using photo reference initially.
4. Learn to draw from a mannequin figure from memory.
5. Do memory drawings.
6. Do multi-figure compositions.
7. Learn to draw independently from the model.