Hello James Gordon,
When ever your sale is decreased this is because of Inadequate traffic or google penalised your site. So you should follow the strategies which gain traffic on your Website.
To gain traffic on your website you have to start marketing your site on different social networking sites. For instance, Facebook, Twitter, Stumble upon, Tumblr etc.However, you also have to take Quality Backlinks from High PR Websites.
If you follow these strategies you will see a boost of traffic on your Website.