Voice-overs aren't like any old speech. They are words that are put together in a script to be read at the same time that a specific image is being shown. They can be a form of marketing (such as a commercial), or they can be a news or documentary-style speech. The way the voice-over speech is written depends upon your goal in compiling the words.
Before you begin writing you need to:
Find your audience, then decide how you want to speak and reach your audience, and you're halfway there.
If you're doing a narrative over a visual track, such as a Youtube-type tape, that's different than a straight audio track, like what you might hear on National Public Radio (which incidentally excels in this kind of narration.)
If you are doing a narration over a visual tape, you need to watch the tape and determine what it is you're trying to convey. Are you describing action? Are you making a commentary on what is shown on the screen?
What you want your audience to come away with from your narrative will determine what you say, and when you say it.
You will have a limited amount of time, whether it's an audio segment or a voice-over narrative for a visual tape, to get your message across.
Sometimes no speech at all can be the most eloquent, and punch up exactly the point you want your listeners and/or viewers to come away with.
No tips on the art of voice-over writing are complete without a discussion of who will read the narrative.
Some narrators can do very well with any type of speech. Many of them may be professional voice over actors. However, if you're using a friend, or a professional who isn't a voice-over expert, be aware that what you write may not come across as you anticipated it would.
You may even want to be your own voice-over actor. If so, make sure you get the opinions of more neutral parties (not your mother!) so they can tell you what it is you really sound like.
Just remember: almost everyone hates the sound of his own voice on a tape, so you may not be nearly as horrible as you think. Of course if you are only asking your BFF or your significant other for their opinion, you may not be doing them, or yourself, any favors. This is the time for honesty! Don't be afraid of it. Get their opinion on your script as well.
Get some friends (or pay some experts) to judge your script.
Use your common sense, and a stop watch, and you'll soon be ready to give others tips on the art of voice-over writing.