dialect

I remember first reading Huckleberry Finn and later Pygmallion. How did Twain and Shaw know how to take the English language and make it sound like it is spoken? From my little bit of work in the theatre I have explored several accents and enjoy learning how to imitate them. However, I am quite a novice at making my characters on the written page sound like they should.
I realized last week that I was getting too worked up about this issue. Until I get more information about the dialect of the region during that time period, I need to proceed writing and then go back later to edit the dialogue appropriately. Especially important will be to reflect educational status in the character's speech. I don't want to fall into class or ethnic stereotypes but remain true to who the characters are as they would have existed in history.
I am hoping to reconnect with my historian who has promised some written dialogue. Currenly, my email and his do not seem to get along, so I may be trying snail-mail. Perhaps slower, but worth it to get this right.