My first question would be, "What kind of story are you writing?" Just because I'm curious.
Now, you say you already have a draft done. That's good. Now read it to yourself, and while you are reading, ask yourself where you think the story REALLY needs more detail. Get a highlighter and highlight those spots.
Look at the parts you highlighted. What kind of things need more detail? Does the plot need more detail? Do your characters need more description? Is you action boring? Are you locations vauge? Read through your draft again, this time thinking through these questions. Highlight anything else that seems to need more detail (I hope you don't mind writing all over your draft

).
Now take another look at your draft. (Are you sick of reading it yet? That's what revision is all about.) It might help to write seperate backstories about certain things, just so
you have a better idea of what's going on. (This is especially helpful for fantasy stories, but is good for any story). Write about a character, a place, a past event, whatever. This might sound like a waste of time, but you'll probably be surprised how much knowing the background will help. It will saturate your story.
Windlord game some great advice for making the action more interesting

But really, it's all up to you.
good luck and happy writing
