An impressive piece of writing is a perfect edifice of rhythmic word collections, proverbs/sayings and phrasal verbs. For the first part, you need to master vocabulary with synonyms and create a cross-connection of mutually sounding words in your mind to use them whenever required. Your ideas should put on the garb of beautifully sounding words leaving lasting impression on the reader's/listener's mind.
I find the following books/audio books extremely helpful in developing an exceptional vocabulary and an impressive style.
1. Verbal Advantage (all levels) (PDF books and audios are available on net)
2. Two-word verbs (PDF book is available on net)
3. All clear! idioms in context (pdf and mp3 files are available on net)
4. Phrasal verbs in context (macmillan series) (pdf+mp3 are available)
5. Word Smart (I&II) (pdf+audios are available on net)
6. 15000 useful phrases (pdf is available on net)
7. Barron's 550 words you need to know (pdf+mp3 available on net)
8. 1100 words you need to know (book available on net)

Try to digest all this material and you will witness a tangible improvement in your writing as you crawl on the lessons.
I invite the input of Babakhan on this because I deem his writing flawlessly impressive.