How do I use the content of my book before I get it published?

Well, I would not worry too much about using your material now. Speaking, talks, magazine articles, etc. are EXACTLY what publishers are looking for when they ask what your author platform is. Most publishers want to know that there is a market behind the person AND the book they are about to publish. If you show that you have an interested audience, and that you have already been published, then a publisher is much more likely to give your book a serious look.

As for your stuff already being "published" I would imagine that you would have to rework any of your material for a lecture different then a magazine article, which is different then the book. Repackaging of one's writing happens all the time, especially in nonfiction. As long as the exact same writing is not republished, then go for it. You'll increase your reputation, your audience, and your potentials for getting published.

There are several ways:

1) Ask them personally. If you are producing quality work, then it should not be too hard to get a few endorsements. Send them an email pointing to your website with some of your work and see if they would be willing to give you feedback. Then find a line in the feedback that you can quote from.

2) If you are giving public talks, I'm sure someone will be happy to endorse your work afterwords. Just ask if someone would be willing to write up a short blurb for you.

3) Get people who have followed your diet to endorse it. If it worked for them, they should be happy to give you an endorsement.

All you really are looking for is a couple words or perhaps a single sentence that you can put as a quote on your website, book, flyers, etc. They don't have to be long, involved endorsements, but simple one-liners that grab peoples attention and give you a credible position in their eyes.