Caveat: I don't know whether this kind of promotion can be duplicated.
There is a service called "Free Book Service" that charges a fee and guarantees that your free book will get up to 10,000 downloads in a few days. The number of downloads depends on which fee you pay. Since they deal only with free books, the service makes sense to me only for a series in which one of the books is free or for a book that is offered free for a few days. In the latter case, you won't earn royalties, but the ranking bump might be worth it.
I purchased the 10,000 download option for the first of my Stanley Bentworth series. Sure enough, I got well over 10,000 downloads (18,000+ at last count). Sales of the other two books shot up at the same time. The free book took a big ranking bump and was #1 in its category for a while. This brought attention to the other books in the series, which accounted for their increased sales, I think.
This promotion did not affect the sales of any of my other books.
Sales of #2 and #3 continue at a moderate pace now, and I picked up a few reviews too. Royalties from total sales following the promotion now exceed what I paid for the service.
The service took a lot of heat when they announced it on Kindle Boards. It's a long discussion (22 pages) and you can read it if you're interested.
A lot of authors opposed the service. None of them acutally tried it. Those of us who tried it were among those who didn't oppose it.
The discussion got locked by a moderator when it went too far off-topic and descended into a heated exchange.