One thing that is indispensible for a writer, as well as for anyone working on standardized test preparation for the GRE or SAT tests, is developing a rich vocabulary.
English is a beautiful language, with a vast store of words and a nuanced range of expression. This comes from its unusual history as a combination of Latin, Old French, and the Germanic and Saxonate languages of the pre-Norman British Isles. Unfortunately, however, many of the great words in English don't get used very often. This is a fact that will be painfully familiar to anyone who has ever studied for a standardized test. These tests usually include a large number of English words that most people have never seen before. To prepare for them, most people need to learn a huge number of new words quickly.
For standardized test preparation there are various methods that are designed to help you jam as many words into your brain as possible in the weeks before the test.
An SAT or GRE tutor can be very helpful in teaching you these methods, which work well and are always necessary to some extent. The only problem with them, aside from the fact that they're stressful, is that when you learn words this way you often forget the words quickly after the test, thus nullifying all the hard work you did. So instead of cramming, why not build vocabulary practice into your daily life even when no test is in sight? This doesn't have to be as tedious as it sounds. If you can just manage to develop a curiosity and enthusiasm for the English language, vocabulary practice will come naturally. You will do it because you want to learn the language, rather than because you have a hoop to jump through, and this will help you remember it for much longer.
You can start by reading widely and carefully, and choosing authors that really excite you.
When you come across words you don't understand, don't just pass by and keep reading; look up the words in a dictionary. Write them down. Go back to the original context and read it again. Use the words in your own sentences. And don't just use a digital or online dictionary either. These tools are useful and can save time, but you would be surprised how valuable it is to flip regularly through a paper dictionary. The reason for this is that when you flip through a paper dictionary you have many opportunities to get distracted and let your eyes wander over other parts of the page, or to remember a word you heard once or a word you looked up before and pause on it for a minute. Just to meander through a dictionary--to take your time and allow yourself to procrastinate by looking at other words rather than rushing to get the job done--will gradually but surely enrich your vocabulary.
As much as you can, try to treat the discovery of English as an enjoyable hobby, rather than a task.
Your vocabulary will develop naturally, you will spend less time forcing yourself to memorize lists of words that feel arbitrary, and you will learn more about a fascinating language. So to that end, here is a quirky little puzzle to get your English exploration started: why does English have separate sets of words for types of meat and the animals they come from? Why do we raise pigs but eat pork, feed cows but cook beef? The answer may surprise you, and finding it out will be the first step on your way to becoming a connoisseur of the wonderful and beautiful language we call English.