I agree with Jeri above that you don't want to get too polished and end up sounding like some formal choral society that has turned its hand to shanties. A certain raggedness and spontaneity has its own charm - think of the Watersons, who were not shanty singers but had this fluid, improvisational feel to their singing. They didn't write it down or work it out formally, they just kept trying until it sounded right.

It did help that they were all related either by blood or by close personal relationships, but unless you all start marrying each other, or each other's partners, there's not much you can do about that!

Marje