My eldest at-home daughter is hip-high in final exams so she has spent most of this week reviewing and studying. Luckily, she's a very good student, so these exams shouldn't be too difficult. She's prepared.
When I write, I firmly believe preparation is everything. I try to know as much as I can before I actually start.
Characters: I write detailed biographies of each of my main players. A great deal of this information will never make it into my manuscript, but I've found it really helps. I'll learn their strengths and their foibles. I'll know their attitudes, their prejudices. Most importantly, I'll know their vocabulary. This makes writing their dialogue a breeze.
Setting: This can be inconsequential or vital, but I learn everything I can about it. I have files of pictures which I have gleamed from the internet. I've drawn a floor plan of the first two floors of the house where my WiP takes place; I've had to out of necessity. It's a big house and I've got lots of characters running about. I know the surrounding area (thank you, internet). Since I know the season ( and the actual date) the story takes place, I've learned what my characters would see growing in the gardens when they're outside and I know whether or not they'd need to put on a sweater.
Time Period: If you're writing something taking place in the past, learn your history. Learn about current events, current fashions, current food. If your story takes place in the present, decide how much you want to make it really now, or if you want it to float in the nebulous 'around now' world. Maybe it doesn't matter, but maybe it does.
This preparation may seem time-consuming, and I admit it can be, but I believe it is also ultimately, a huge time-saver. Doing this work will make your world real. You'll know who your characters are and where and how they live. Once this is done, once your world is built, you can toss in your plot with all its troubles and crises.