I recently became concerned that my website was losing ground to some pretty poorly optimized competitors, seemingly out of the blue. After some panic, I realized that this was because the search results I was being served were majorly skewed by my search history and browsing history. When I turned off cookies and went through a proxy, my site was in the same position it had been all along for most of the keywords. It's easy to forget that Google shows a different set of results to each user, based on the usage profile stored for that user. I feel you can get a more objective sense of where you stand in the SERPs by turning off cookies and going through a proxy (you should use a proxy within your home or target country). I believe I was being served wildly different results because I had spent so much time trolling through the pages of my keywords' SERPs and clicking on my competitors, even if they were buried deep in the results. This told Google that I found pages relevant for different reasons than it normally would've expected. In my case, I believe I trained Google to believe that I preferred results in the form of blog posts, as 3 of the top 10 positions changed to blog posts, even though an anonymous search returned results with no blog posts in the top 10. That's my theory. Anyways, just remember: don't freak out if you start to drop in the SERPs if you're checking using your home computer!