Voice overs have a number of uses, but there are a few things to stay away from when trying to record a great narrative track.
Voice over work can be important for many video projects, especially documentary films that are often unscripted and need to be given the skeleton of a linear flow. Voice over can add character and correct progression to a film and can be the most important factor in keeping an audience’s interest. This form is still difficult for most people new to that type post-production, especially since many of the techniques are counter intuitive.
The first thing you have to remember is that you want to avoid any built in microphones on computers. It is easy, and often inexpensive, to go ahead and get an exterior microphone to plug in to your computer to get better sound. You have to have a quality microphone to get this sound because it will be too distracting to listen to if there is any outside interruption. If at all possible you should use a digital sound booth to get this.
You have to avoid people who have trouble reading out loud or have irregularities in their speech. The voice over is not supposed to draw attention to itself usually and if there are any mistakes or odd voice uses in the track then it will become very apparent. Avoid using anyone with an unusual accent, speech impediments, or trouble with reading at an appropriate pace. Those qualities are just difficult to work with in this specific setting. Avoid non-native English speakers for this situation unless you are trying to highlight a foreign issue. This is just because people will almost always pick up on this and then focus on the voice patterns rather than the content of the speech.
Avoid using long and complicated language during voice over. Try to use easily heard words and keep sentences fairly short. Try to stay away from awkward words that are relatively unknown to most of the population. Use power words to maintain the flow of the script, avoid passive voice, and keep consistent in your use of accents and pacing.
Try to avoid repeating what interviewees have already said if you are working with a documentary project. If you end up including both you will just be reiterating the same point and it will become redundant and boring.
Avoid recording audio without any headphones available while you are doing it. Without this kind of isolated sound back you will not be able to accurately hear what the audio ends up sounding like uninterrupted.