Most of my experiments have been with a book aspect sized image, but not worrying about making it look rectangular. So a white background and just stick within the confines of that image
Actually one of ASparrow's books is a good example of this -
(I like this cover btw)
http://www.smashwords.com/books/view/20932
I think part of my problems is I am too judgemental of my own work. It does help with re-writes though.