Quality writing always requires practice? Correct.
The practice can only be done through reading piles of complicated books and learning difficult vocabulary words? Wrong.
1. Try to speak more formally
We know how to brush our teeth without thinking because we do it twice a day, every day. Perfect practise makes perfect. This is why native speakers of English can still find formal English writing difficult because people rarely speak as formally as they write. Sentences sound colloquial if the way words are arranged are mixed up or if some words are cut out.
e.g. The students were mad about cuts to funding.
VERSUS
The students were mad about the cuts to the school's funding
e.g. Can't the students just complain to the government?
VERSUS
Can the students not just file a complaint to the government?
If you try and speak using proper grammar and sentence construction on a regular basis, formal tone will come to you more naturally and your writing will quickly see improvements too.
2. Actually read through grammar rules
Poor grammar tends to be habit. If you are a native speaker of English, chances are you learned how to put sentences together through copying how those around you put sentences together. As you repeated word arrangements over and over, it began to sound "right".
However what sounds "right" is not necessarily grammatically correct and your grammar habits can often lead you into using incorrect tenses, prepositions and articles. This is why proper knowledge of grammar is the only way to truly be correct at all times.
3. Re-learn the vocabulary you already know
The use of the precise vocabulary is the most important difference between sounding professional and sounding confused. Precise vocabulary means to use a word that means exactly what you want to say. This means you need to know the actual nuances of words instead of just the general idea of the word. This means that knowing the vocabulary words you already know in a precise way is just as effective as learning whole lists of new, longer words. Avoid the use of phrasal verbs in formal writing. What you can say in one word should never be said in more.
Phrasal Verbs: don't say blow up, say inflated ; got up = rose ; mixed up = confused