Here are a few “gimmicks” I’ve received over the years from people who mailed their book proposals to our office:
Cover letter printed on fluorescent paper
A twenty-dollar bill inside the SASE
A wooden treasure chest full of chocolate gold coins
A signed photo of basketball player Michael Jordan
A rock (in a proposal about God as our rock)
An 18″x24″ piece of artwork, on a presentation board inside a larger portfolio carrying case
Does it work? Not really. Gimmicks end up being used as writers conference fodder for what not to do. Realize that your proposal is more of a job application. You want your “résumé” to look professional and competent. A publisher is “hiring” you to do a “job” and will pay you to do it. So keep it simple.
Since email has taken over much of the submission process, the gimmicks have decreased, thankfully.
And, no, I didn’t keep the $20; it went back to the writer in their SASE, along with a rejection letter.