Can you do an Irish accent? Or Scottish? If so, think of that ("rhotic" - emphasis on heavy "R"s at the end of words) and then flatten it out. In other words, none of the lilting sing-song type quality they have. Rent some old classic American movies too - especially Westerns. Being "Of Mice and Men" is set in the Depression and is about migrant workers coming to California they most likely would have had a Southern Midwest accent (Texas, Oklahoma, Missouri). I think its easier for English people to do a Southern US accent than the standard for some reason.