book covers: what works?

I have become a stalker of book covers, lurking about in Chapters, studying, assessing, taking mental notes. After all the work I have done on the novel itself, soemtimes it feels as if it has all come down to this -- a ten second encounter with a potential reader who, like me, is scanning covers trying to decide what to pick up. Lulu offers cover design formats, but I am thinking of putting a little money into a designer. Two covers I liked -- Sharp Teeth, by Toby Barlow, and Ladykiller, Charlotte Gill. Strong colours, single arresting images, minimal approach. I guess I am developing an aesthetic preference. This too is helpful, sharpening how I want the book to be understood, the tone I am trying to project. With each day I turn the prism of the book in a different direction, sensing all the ways I can connect with my readers above and beyond plot and characters.