March 24 - Ask well-known authors or public figures who are relevant to your topic to endorse your book. This may seem like an impossible task, but really all you need to do is ask -- you never know what they'll say. It could be yes (more often than you can imagine)! You might be met with resistance sometimes -- maybe they'll tell you that they don't want to put their name on a competing product, but just kindly remind them that they are putting their name out there again as well -- you'll simply be supporting each other for the same cause.
When do you have enough endorsements and testimonials? Never. Never stop asking for feedback (which is what an endorsement is).