It might help to take a few examples where your writing was good and see if you can identify what helped in those cases... then, consider some not-so-good examples and see if there is something present that was absent in the good cases, or something that was absent but which was present in the good cases.
You can also try looking at what approach / style works for you - an outline of the structure of the argument to be made, for example, or dot points of material to cover, or ...
I have found that I write much better when I know what I want to say - when something is unclear in my mind, it tends to come out as unclear / unpersuasive / muddled etc on paper. When I can modify / polish, one thing that I look for is lack of clarity in my expression as it usually signifies a need to reflect on what I am trying to say before attempting to edit it.
Also, it is worth remembering that if you attempt to polish a turd, the best you can get is a polished turd - which is still a turd! In other words, look for big problems first. If the work doesn't flow because it is poorly structured, fixing the logic / sequence is far more important than fixing the writing. If you do the latter first, you still have to reorganise / add / remove, etc, and the polishing of expression will likely need re-doing.
For exams, where editing is not practical (largely), once you recognise the circumstances that help you to write well, use those to your advantage. If, like me, you write better essays if you have decided on the position you want to adopt, then it may well be worth considering for a couple of minutes, maybe jotting notes, rather than diving straight into writing. I think it is easier to write creatively from a starting idea and allowing the ending to develop as you go than it is to write a persuasive essay by that approach.
Finally, note that there is a danger in approaching an exam question with an essay already in mind. Some people are skilled at shaping and adapting a planned essay to a given question (or are lucky that their plan matches the question asked). Unfortunately, a pre=planned essay may be ill-suited to the question, or an attempt may read like trying to jam a square essay into star-shaped hole. The result is neither effective nor particularly persuasive, and a marker thinking "this is a really nice answer to some question... it's a pity that question is not the one that was asked" is not going to give a high mark for answering an unasked question.