There's no hard and fast way in, but it's a good idea to get as much 'voice' exposure as you can. Write to all your local TV and radio stations and offer to do some unpaid work experience, even if it's not initially doing any voice over work. Also, don't forget hospital radio as a way of gaining some experience. Oh and don't forget to send in a tape/CD of some of your 'work' to these people. If it's purely voice-over work you want to do (rather than this being a stepping stone to doing something else), then the work itself can be irregular and working unsocial hours; you are often called up at very short notice to cover. Like all 'media' jobs, hands-on experience counts for everything and it's often who you know and not what you know, so go and schmooze some local TV/radio exec types. Good luck!