There are a few rules that good writing must always follow.
1. Firstly, make sure the grammar is correct. If it is not, then for sure the article/cover letter/resume will not see the light of day again.
2. Secondly, ask yourself who your audience is - who the article is for, or targeted towards. An article targeted towards a university lecturer, for instance, would be very different to that targeted towards a peer-group.
3. Make sure what format is required for referencing, and stick to it. Case studies in Law, for instance, have very specific requirements for these.
4. Match the style of the writing to the purpose for which it is being used. For reports, and other documents, ask yourself if a paragraph could be better explained in bullet points instead of lengthy prose.
5. Do not make your sentences too long.
6. Do not explain in lengthy, unnecessary detail if not required
7. Make sure you stick to the subject that you are writing about. It is OK to diverge from it, but only for relevant issues, and make sure yuo return back to the subject.
8. We all like to write about something that we are interested in. However, all the writing that you do will not be in a subject that you are knowledgable about. So, always do your research - and quote the sources.
There are very many and varied styles of writing. You should try to find your own, unique style, which encompasses all of the above, and still gives you freedom of expression. Write as much as you can. You can start a diary, and write down your thoughts for the day, and experiment with styles and forms of writing. Read as much of others' work as possible - this will not only improve your grammar, but also you expression and style.
Well, I hope I have been able to help you. If you need advice on specific work, please email me, and I will try to respond within the shortest time.
Thank you for stopping by, and happy writing!