The authors of Value Proposition Design provide ten prototyping principles. I'll share details for a couple principles.
1. Make it visible and tangible.
2. Embrace a beginner's mind. Prototype what can't be done. Explore with a fresh mind-set.
3. Don't fall in love with first ideas -- Create alternatives.
4. Feel comfortable in a liquid state. Early in the process the right direction is unclear. Don't panic and solidify things too early.
5. Start with low fidelity prototypes.
6. Expose work early and seek feedback.
7. Learn faster by failing early, often and cheaply.
8. Use creativity techniques.
9. Create "Shrek models." Shrek models are extreme or outrageous prototypes you are unlikely to build. Use them to spark debate and learning.
10. Track learnings and progress.