Most authors have gimmicks nowadays. Tom Schreck has beer cozies, J.A. Konrath has coasters (I'm sensing a theme here), Joanna Campbell Slan has scrapbooking, and Luisa Buehler has coffee. A platform is a theme for what your book is about; a gimmick is something you use to market that platform.
Two questions arise from this observation: what makes an effective gimmick and do gimmicks actually sell books?
An effective gimmick will help you remember the author, or better yet, will never let you forget them. At last year's Love is Murder conference, Jamie Freveletti (a black belt in Aikido) and Tom Schreck (a trained boxer) performed a fight scene demonstration. The image of this petite woman bringing a six-foot tall boxer to his knees is forever burned in my memory, which makes it an effective gimmick.
Giveaways like coasters and beer cozies can be effective because they're things you'd actually use. I still use my BIG CITY BAD BLOOD notepad and my Jack Reacher USB which looks like a Swiss army knife. Pens are kept but seldom looked at, bookmarks and postcards can be easily thrown away, and anything high tech and fancy will be expensive. The items which are used and kept are more effective because people will see your name everyday and, like in the case of my notepad, others will too.
Though effective gimmicks work well to brand an author and increase their name recognition, will it translate into sales? I've never run out and bought a book because someone gave me a cool pen or a snazzy matchbook. Bookmarks tell me a bit about the book and about the author, which could theoretically lead to a sale. However, the fight scene demonstration definitely sold some books.
The difference is personality and author recognition. A pen or bookmark picked up from a freebie table won't directly sell books. A pen or bookmark given to you by an author with a few minutes of conversation might. A unique demonstration or moving speech is likely to translate into sales because readers get a glimpse of the author. A glimpse of the author is a glimpse of their writing, so the more readers know about the author, the more likely they are to read the book.
Not every author has a platform and not every author will have a gimmick. But if something sets you apart, if something differentiates you from others, find a way to utilize it. If you're unique, you'll stand out amongst a crowd of authors, and so will your books.