Unfortunately, I've yet to have any professional experience. I have, however, been doing it since I was 5 years old, roughly 15 years. Anything from mimicking the voices of Yoda or Mickey Mouse to random alien or Pokemon-like sounds. I've also been mimicking accents for years, and have obtained quite a few I can do easily.
The only actual training I have, was the two years of high-school drama I took.
Anything else, I've done myself or with my friends and family for the past 13 years. My mother and I would often have conversations in random accents or voices, which contributed to my ability to speak fluently with many of my accents. Me and a few of my friends would also joke around with accents, strange voices, or our best impersonations, my personal favorites were always Yoda, Darth Vader, or Bill Clintion.
Aside from that, anytime I watched my favorite shows, I would often mimick the voices of my favorite characters. After learning the episodes or movies by hear, I'd turn the sound down to the point where I could just barely hear them, then say their lines when they spoke them, just to see if (to my ears), I got them down to a T. Anytime I deemed myself not good enough, I practiced imitating them some more.
Also, though it might not be considered proper 'training'... I have a tendancy to break into random voices or accents, depending on my mood. As well as, when I write, I tend to give voices to my characters as I'm writing.
I write, a lot, and have a large imagination. This enables me to find good options for the voice and attitude of characters or creatures when it comes to cartoons or books... When the client ha something specific in mind, I am usually capable of sliding easily into or out of a specific 'role'.