Principle #1: showing is better than telling.
-  You think you can do XYZ.
-  You’ve done XYZ in the past.
-  You’re currently doing XYZ, and doing it well.
Principle #2: external validation is stronger than your personal opinion.
-  You claim XYZ.
-  Numerous people who are affiliated with you (friends/accelerator batchmates/etc.) claim XYZ.
-  Numerous people who are completely unaffiliated with you claim XYZ.
Principle #3: more data is better.
-  Your product has 0 sales.
-  Your product has 5 sales.
-  Your product has 50 sales.