Any formula tends to get tiresome after a while, but one could do a lot worse than to follow the step-by-step query letter plan from this post on Writer Beware.
Fun fact: when I sell a book to a publisher, parts of my original submission letter to editors often end up as part of the flap copy or the back-of-the-book copy.
Even funner fact: particularly when I'm selling an author's first book-- the one that author originally queried me on?-- I often "borrow" large chunks of the original query when I'm putting together said submission letter. It got my attention, didn't it?
In other words, there's a decent chance that your query letter could end up on the cover of your book. But, y'know, no pressure to make it good or anything! Hah.
Can you give an example of cover copy that works? What sorts of descriptions make you want to buy a book, or check it out from the library? Can you learn anything from those examples?