Write realistic dialogue. While dialogue doesn't necessarily have to be written exactly the way real people speak (real people use "uhs" or "ahs" for pauses, for instance), it should be close enough to capture the verisimilitude of real speech. Examples: People often contract their verbs and use colloquialisms or incomplete sentences.
Use realistic settings. Unless you're writing sci-fi or fantasy, make sure the settings are as approximate to the world you're creating as possible. If it is in an office setting, include details that are familiar. For instance, your office workers might use Mac computers, or the office manager might drink out of a mug with the company's logo on it.
Be accurate in your description of your character's job responsibilities. Do your research. Find out what the job involves, the procedures or skills one performs on that job and the people likely tol be his or her superiors. This is true even if you aren't writing a procedural. Research any work responsibilities (office supply manager, restaurateur, mechanic) or hobbies (sewing, gardening, horseback riding) that require specific knowledge of skills or procedures.
Use the correct nomenclatures. Nomenclatures are simply the names for things. It helps when describing certain objects in your story to have the correct names for them, especially when dealing with something such as human anatomy or architecture.
Use details that are specific to the region or area of your fiction. Determine, for instance, the type of trees, animals, weather patterns or architecture that are native to that area. Slang, jargon and dialect can also be regional. Even the names of junk food or fast food franchises will be different in certain regions. If you're not from the region you're writing about, do your research.
Use pop cultural references. This, of course, is a matter of style, and some writers loathe using pop cultural references in their writing. But, if used wisely and sparingly, they can create a certain verisimilitude to your fictional world because they are so readily familiar.
Avoid anachronisms, especially if you're writing historical fiction. Study up on the period you're writing about and research the period clothing, jargon, foods, technology and inventions to make sure that you don't include something in your story that hasn't been created yet.