1. Who is your audience?  Answering this question will help you find out what a potential reader might expect on your cover.  Sure, some covers are shocking or risque, but if they look at your cover and don't know the theme or subject matter within a few seconds, they may not know they're interested.
2. No clutter!  Cluttered covers bombard the viewer with too much information.  It may tell a story to you, but the reader's eyes will be all over the place, and your sales pitch will come off garbled.
3. Use easy to read letters, and do not use a lot of different fonts.  This is basically a subset of tip #2.  Sure, having two fonts are fine.  The main line could be one font, and then you can switch to another, more stylistic font for a single word to add flair.  For instance if your book were entitled: Delicious Dinners in 15 Minutes, then changing the font for "15 minutes" would be a great way to emphasize that the book is filled with quick recipes.  If your potential buyer identifies themselves as a busy person, then you might have a sale on your hands.
4. Your cover should tell the story of your book.  If it's a murder mystery, I want to see someone tracking someone down or a murder happening.  If it's a thriller, I want a cover that says "suspense" and "action," with people chasing each other down dark alleyways with guns.  If it's about spelunking in ancient Mayan caves, I want to see some Mayan architecture.  The theme has to be apparent and at the same time exciting. 
5. Don't pick ugly colors.  There are pretty shades of green and there are disgusting shades of green.  A book cover shouldn't grate on your viewer's eyes.  I've seen covers that had a number of strong points, but then the designer felt that they wanted to surround their work with magenta, and instead of focusing on the great images they picked, I felt that someone had let a magic highlighter loose all over their work. Simplicity works best, and your colors should mesh.

Here is a writing sample from my fantasy novel I've been tinkering away at for a few weeks now.  I'm trying to tighten my style.  I always worry whether or not my plot will be gripping, like I have to find some grand theme that will explain the human existence.  Then I'm reminded that some of the most popular, and most effective stories were simple and fun, and still portended to larger issues and had just as much value as something as heavy as, say, Crime and Punishment.  Any critiques/comments/shout-outs would be great!  My email is yithril@gmail.com.
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Here it is, the introduction-

Here is a stream of consciousness exercise I like to do if my brain gets jammed and I suffer from terminal writer's block.  For me, I write in bursts.  Sometimes I feel invincible, like my next book will break new ground and be beloved by millions (I have an active imagination), which gives me the fuel to write for hours.  On the other hand, sometimes I have zero motivation and find that I surf youtube for hours and write maybe a sentence of my story. Then, when the day is over, I feel like I've failed, for not being able to push myself to write even one page.  While you cannot dictate the creative process, you can refine it with some discipline.  This stream of consciousness method can work.

Describe a room in your house and write about it for 25 minutes straight, without stopping.  Forget about form, forget about plot, forget about spelling and grammar!  Picture yourself setting your work ablaze (or just plain deleting it).   Discipline is built bit by bit.  This is something I learned in Korea.  If you do this regularly, you'll find yourself able to write longer and with more quality, rather than spending your afternoon checking the baseball game score.

Here's an exercise that we did once when I was taking a class on translation, and I thought it really helped in being mindful of voice and how to switch it to convey different information.  The idea is quite simple. You can do this exercise if you're working on a big project, like a major paper or a book.  For the class, we had an extensive 20 page paper due by the end of class on a topic that we had been steadily researching.  


Picture that you're in the following situations.  Then, try and explain your novel/idea to the person in these settings.  Try to sell them on your idea and communicate.


1. Meeting an elderly woman on the bus.  By chance you and she strike up a conversation after she drops something, or someone bumps into you.  You tell her you're an author, and she asks you what your book is about.


2. You and your best friend are chilling at a bar.  The music is going, people are all around, and you and he/her have a moment where you can really talk.  "So, what's this novel you're working on?" he/she asks.


3. You're in a board meeting with some imposing looking executives in full business attire.  You've just shook hands with each one, and you're thankful that your connections with the secretary got you this meeting at all.  They're interested in your idea/novel.  How do you sell it to them?


This not only helps with say, book descriptions for product listings or teasers for your work, it's also a great way to stop and ask yourself: "What's my voice?"  What tone do you take when you write and how would switching it up change your style?