Voice-over, narration, and generally non-acting voice work is one of those things that seems easy, but the people making a living at it have real practice in their craft. It's not enough to have a "nice" voice, it also requires a command of language, ability to do research, possibly some linguistics training, and definitely extensive practice.

For reference, I have been a high school English teacher and radio announcer (you find a lot of people in voice work with backgrounds in language). It is not enough to speak clearly, you have to have something "most" people do not have. Nearly anyone can record clear audio with a decent setup and/or digital editing equipment. But now we're taking about voice, and language, as instrument, not as tool. The difference between a competent pianist playing a steady nightclub job and a world-traveling virtuoso performer is not the quality of the piano they use (their tools), it is the degree of artistry, experience and dedication they bring to it.

For example: When a good friend of mine in high school was deeply saddened for discontinuing her piano lessons I was confused. I told her she should be happy, after all now she didn't have to spend 3-5 hours a day practicing, and she'd have a lot of free time. After all, she'd been playing classical piano for 12 years. What she was sad about was basically giving up piano though, because the next step for her would have been music school and that "virtuoso" track, however it actually shapes up.

You need to treat voice-over the same way. Having a nice mic, good software, and a pleasant voice allow you to get started. Now you're looking at the same 3-5 hours a day of practice. Not just talking. Learn about linguistics, learn about inflection, learn about the psychology of language, and about the business you're in. Understand in great detail, more detail than your customers do, what they want and how to provide it. You're the expert, it's not up to them to know what's "good enough," it's your job to deliver excellent, every single time. That takes practice, knowledge, and more practice.

Start small, just like anything else. Offer your services free or for very low fees. Announce little league games or high school sports. Narrate films for college production classes. Build a resume, get your name out in local circles. Most people in media make the mistake of expecting to succeed too quickly. It takes years and effort, and more years and a whole lot more effort, to build that up even for the very successful people in the industry. Get a good group of people you work with, maybe dozens or scores. As they grow and make contacts in the industry, you'll grow along with them. Their projects become larger, and by association so do yours. Working in media is far more about networking, meeting and knowing people, and taking opportunities when they arise, than it is about any specific set of tools. But if you're the best prepared, the most capable, and you know how to deliver excellence without being guided to it, then you get called again, and recommended. And so it goes on up the ladder. But this is a years-long undertaking, not a few months to a steady paycheck.

For a little more information on the sorts of things you should work on, here's a long answer I gave just about avoiding verbal "ticks" like err, umm, etc.

http://www.mahalo.com/answers/mahalo-answers-community/how-do-you-stop-from-saying-um-er-ah-and-so-when-youre-talking

Good luck!