Sometimes the best writing is birthed over time.
I think a good writer needs to have a solid amount of life experience before they start writing because in order to create genuine characters, these characters must be empathized with, which means you must have a certain level of familiarity with the things they are going through. Here is where the crazy writing cliches originate from, like sitting in a coffee shop for 8 hours watching people, or crashing funerals to see what death really looks like.
Artists excel in this style of research. Many I know take anatomy classes just so they can learn the muscle groups in their subjects faces and therefore produce more realistic work.
So when I say that good writing comes from life experience, sometimes being a good writer out of college just doesn't happen. Sometimes you need to get out and work a shitty job to get a raw sampling of emotion. In this blog, however, I will give some tips on how good writing can be produced out of life experience.
Certain people just like to swear. I never really endorsed this style for the reason that not all people swear as much as literature would make it seem. Generally swearing is reactionary, like when the character is scared or startled, or hits himself on the hand. Moments like these are places where use of profanity can help to build a realistic character. Thinking about what your friends are like and how they swear will help to produce realistic characters that don't speak like stereotypes.
Another thing I do is pay attention to the mannerisms of people in retail stores. The Apple Store is the best, because in my experience there we find lots of pretentious assholes who make very animated expressions. Now, putting this in your book is a little different than the previous example. What I want you to do is to note these expressions, and then try to describe them. The better you get at doing this, the higher the likelihood that you will be able to execute good character descriptions and mannerisms from real life.
Lastly, my favorite exercise is watching opera, or anything staged in a theater. A good actor must over emphasize their movements on stage, that way from a distance they can appear more lifelike. This is because when you are hundreds of feet away, it's hard to distinguish the subtle emotions of someone you can barely see. Getting to know these lively displays of gestures and actions will help you better describe what characters are like from real life experience, because just like acting on a stage, a reader will only explicitly detect what you make explicit.
That being said, happy writing. And if you enjoyed what I've written, refer a friend to my work, or keep posted for more!