I found a book at Amazon that may offer some help.
The Self Publishing Toolkit
It costs $3.99 for a kindle file of the book and among a lot of other things (such as designing promo kits) it offers tips on how to design a good cover and what to avoid while doing so. It is geared to a certain extent to publishing at Amazon, but I imagine a lot of what is offered in the book applies to any self e publishing effort.