How to get started as a voice-over artist

Pitch pitch pitch! Have a go for every casting within your skill set and each little sample script and performance will form part of a great little portfolio.
To begin with I pitched to students then professionals and even found great feedback from what I thought were weaker area's of my range.
I have a small studio set up as many castings are available to do from home (travel is extortionate at short notice as some jobs tend to be a quick turnaround!) and this can be done reasonably cheaply.
I also listened to other VO actors reels to see what they had that I could learn. Record your idea's! I have a fair few engrained in the memory but document what you do then you may be able to call on it at a point.
Offer your services too, students projects although low to no budget can form again great pieces for your portfolio and can show your range.
Even when I cast for a role and was unsuccessful I always listened to the final job (where possible) to see where I may've 'done better'.
The internet of course can prove to be useful for tutorials, articles etc.
Once you are confident of your reel and that it shows you on form then pitch to agencies, usually a call or email will prompt a response and politeness always gets you the answer (even if it's a no!).
Be professional! Dress for the job you want, imagine you're up against the premier VO artists because in some cases you will be.
Be tenacious and be more tenacious!
VO industry is extremely competitive but anyone with the confidence to get out there and do it I predict will generally have something to offer. I can't do corporate/smooth and fairs fair that takes away a large area of work but I accept this and go for as much as I can in my area.
best of luck