First, be able to summarize your entire book in no more than four sentences. If you can do that, it shows you know what you want to say and where you're going.
I don't know anyone else who does this, but on page two of my proposals (following the title page), I write what we call a précis statement or the elevator pitch. I put it in a box with a 14-point font.
From reading those three or four sentences, editors can easily decide if they're interested. If not, I've saved them a great deal of time and they can stop.
Second, consider the proposal is a soft sales pitch. Don't claim that it will change lives or it's the best novel written since 1806. Present the book honestly and simply. Editors probably know better than you what the book can do.
By contrast, a self-published writer tried to get me to buy a copy of his book by telling me that "everyone who reads it is instantly changed." That statement also appeared on the back cover. (His best friend told me that the author had sold a total of four copies.)
Let the manuscript sell itself. And the soft-sell pitch is your enthusiasm. I don't write anything unless I'm excited about it. That excitement shows in my writing.
I don't like pitches that all but guarantee I'll become a new person by Friday, permanently lose 20 pounds in 14 days, or find a soul mate in three months.
Finally the essential element of a good book proposal is one that clearly expresses your well-conceived idea.