Book covers are important. It's like a greeting card for your novel. It draws your reader in, and tells him at a glance what sort of story to expect.
While looking at tons of covers in the bookstore/Amazon, I noticed a trend in covers, a trend I thought would help you. So here you go: a quick and dirty guide to covers.
Science Fiction: Spaceships!
Space Opera: MOAR spaceships! Preferably near a planet or asteroid.
Cyberpunk: Dude in front of six computer monitors, wearing a blu tooth headset.
Dystopian: Picture of a charred wasteland.
Steampunk: ALL the dirigibles.
High Fantasy: Dragons over a pseudo-European forest *If your high fantasy is the one or two books that takes place outside Fake Europe, it will be appropriately ethnic.
Urban fantasy: badass chick or dude leaning against something. If your book features more than one sex scene, the chick will be wearing all leather and displaying brokeback.
Romance: Sexy people time!
Contemporary: Sexy dude with abs and sexy woman with boobs half naked about to jump each other.
Paranormal: Same as contemporary, only dude will have fangs and/or yellow eyes to indicate he's NOT HUMAN
YA: Teenagers! Dressed in teen clothes!
Contemporary: Pictures of lakes, boats, schools, and food. Lots of food. Teens like food, right? Put food on the cover and they'll buy the book.
Paranormal Romance: Same as above except everything is DARK and BROODING
Literary: Landscapes and pretty scenary!
Old White Dudes Having Existential Crisis: Artistic watercolor rendition of a city
Middle Aged Woman Fighting for Her Place in Society: Garden Scenery with Birds
Horror: All the dark colors! Houses! Desolate woods!
Historical: Female in period-appropriate clothing standing in front of famous landmark
Clearly this is not an exhaustive list, but hopefully it will point you in the right direction when it comes time to chose or offer imput on your book cover.
Do you have any other suggestions for genres? Plop them in the comments section!