Momentum after a successful campaign.

Ran a really exciting campaign on our pulp killer thriller Nightingale Vale, if slightly short lived. Our recent free download resulted in our most successful sales yet for frees nearly 4,000 downloads resulting in a flurry of activity making us peak at 82 in the Top 100 for psychological thrillers in the States and resulting in a torrent of sales. This is interesting on a lot of levels. What made the campaign so successful? Can it be repeated and how can we try to maintain the momentum? We would welcome the views of any authors who have run a similar campaign to see if there is a common theme that would enable the success to be maintained. We shall see. All views are welcome.