Pick up any reading material.. a magazine, newspaper, anything.. and just read it in a different voice. Create a character in your mind with an accent and just read it as that person. Then start having conversations with other people you don't know, introducing yourself as that character. You'll have to invent life details on the fly, but that's fine as long as its consistent. Do this for about a year.

It's really the acting part that matters in the end. :D