I'm in the process of collating top tips and resources about designing ebook covers. I've compiled a list of useful links (see below), which I'm adding to continually, but if you have any advice based on experience it would be interesting to hear about it.
There's a brief questionnaire here.
For myself, I'm not totally convinced that ebooks need covers (some Kindle publishers don't include it anyway). However, I tend to create them anyway, usually using a combination of Flickr, Word and Paint.net.
But I'm sure there must be better options, especially for arrtistically-challenged people such as myself!