Take several acting classes. This should actually be TIP #1, but I saved it for now because while many people tend to think that it's all about the voice, once they learn that they also need to know how to act, they can get discouraged. I certainly did. I remember thinking, 'What? Just having a great voice isn't enough?" In some cases it can be. However, the majority of the time, simply having a great voice is only part of the equation when it comes to building a solid career.
It's okay, and it really is worth it. If you truly love voice overs, learn to hone your skills and bring a script to life by learning first how to act. It will bring your voice over performances to a whole new level.