As an add-on to the dialogue post, take expressions into consideration when writing for characters. Consider your character’s environment, age and socioeconomic status. A socialite heiress wouldn’t be using street slang, and a character in California or New York won’t be using expressions from the deep South. Time periods are also important to consider. Online searches can easily dig up historical slang, from the age of Shakespeare to the Roaring Twenties. For modern day slang, just listen to the people around you or look online in places like Urban Dictionary. Remember that the expressions your grandmother uses are not the ones your friends are using, and that expressions differ across cultures (also keep in mind that characters would have trouble with idioms in other languages, which can be used to comedic (or dramatic) effect).
Wordsmiths 