Style, tone, grammar, punctuation, genre, rhetorical devices - an excellent writer must master these 'tools of the trade' to achieve the desired effect in any piece of writing. Consider the types of writing you most enjoy reading - what makes them good? What if they were written by other authors - how would they be different? Take time to notice the techniques used by writers and try to achieve similar effects in your own writing by experimenting with them.
Here's a quiz on the subject to get you started.