I prefer to have a teaser paragraph written by the time the first draft is well begun, so this book already had one when revision time came. So much had been added to the plot by then that the blurb needed redoing, though, and actually it still needs work as some version of it will get used in my query letter and I want it perfect. Grr. It's harder than it looks.
Anyway, there are several benefits to having your ideal back cover written before revising your novel. It will force you to know your hook* and the most interesting and relevant plot points, as well as firming up your concept of just what kind of book you wrote. [That last point is more important than it might look: the reason I had to re-revise half my story is that the first and last halves didn't match. They read like different genres of fiction. Ah, learning experiences.]
Best way to write a back cover blurb, as far as I can figure: Write down your basic hook* and list the key occurrences in your story. Referencing those, compose a paragraph or two that captures the basic appeal of the story without destroying the suspense.
* The word "hook" is used in different ways in the writing and publishing community, but I'm using it in the sense of eye-catching premise. For instance, the hook for Lord of the Rings might be something like "Peaceable little fellow inherits a magic ring that turns out to be the source of power for an evil ruler, and must dispose of it right under the Dark Lord's nose ... er, eye."