It is all too easy to spend a lot of time on a writing assignment without the effort put in earning a high score. There are key things to check to maximize the chances of success.
It is essential to stick to the task set and not drift into irrelevance or, even worse, set your own task and do that instead. So, be relevant.
Plan any lengthy piece of writing if you can. Consider the assignment carefully and try to break it down into the constituent issues which need to be dealt with. Make rough notes or jot down key ideas and rank order them in the order you need to write about them. Think carefully about the order; remember you are aiming for the most logical progression. At this stage it is easy to change your mind if you need to.
Avoid repetition unless you specifically wish to use it to sum up or conclude. In other contexts it can look like muddled thinking.
Pay attention to any word limit set. Verbosity seldom earns credit.
Consider whether adequate weight is given to every part of any assignment; writing too much about one aspect and too little or even nothing about another is a common failing. Aim for a balanced response.
Last but not least, allow enough time for the assignment and make sure there is a congenial environment in which to work.