Cover Junkie have just introduced an awesome new aspect to their site. You can now create your own account and save your favourite inspiring covers to this. Sweet.
I think the most important thing for a cover is that the image on it stands out immediately, that it is striking in some way. The second thing I think about when choosing a cover is how well it explains our theme.
Fashion magazine i-D celebrates 30 years of fantastic covers in this book by Taschen. The iconic magazine, complete with it's winky eye logo, features cover models winking / covering one of their eyes. Pretty damn clever.
Coverjunkie is always a good source of inspiration. The site archives iconic and award winning magazine covers - there's even a category dedicated to Kate Moss (she's a busy girl).