Don't say lay for lie; lay expresses action; lie expresses rest.
Don't say them bonnets; say those bonnets.
Don't say party for person.
Don't say it looks beautifully, but say it looks beautiful.
Don't say feller, winder, to-morrer, for fellow, window, to-morrow.
Don't use slangy words; they are vulgar.
Don't use profane words; they are sinful and foolish.
Don't say it was her, when you mean it was she.
Don't say not at once for at once.