Buy a textbook or grammar book. Sitting down with an actual grammar book and doing exercises is the method of choice for schools everywhere and can certainly help you improve your overall grammar. If you need a major refresher, pick up a high school grammar book to review the basics and essentials. If you want to perfect your grammar, try using a grammar book that's specifically designed for adults. You can pick up adult grammar workbooks at your local bookstore.
Keep a vocabulary notebook. Each time you learn a new word, write it down. Include the part of speech, definition, various usages, synonyms, antonyms and your own example sentence. Keep your vocabulary notebook on you at all times; if keeping an actual notebook isn't possible, create a text file on your phone that you can regularly update. When you read books, newspapers or magazines, underline any new words and add them to your notebook/file. Review all your new words regularly and try to work them into your conversations and writing.
Spend extra time on homophones, also called homonyms. Some of the most common grammar mistakes adults make are confusion between words such as "there," "their" and "they're" or "complement" and "compliment," which sound the same but have different meanings. You can be seen as uneducated or professional when you confuse homophones, so taking the time to learn the clear distinctions between them can help your career as well as improve your grammar and overall writing.
Sign up for a word of the day. Many online dictionaries, including reputable ones such as Mirriam Webster, offer a vocabulary word of the day for free. The word of the day usually includes the definition, part of speech, usage, synonyms and antonyms. You can sign up for most word of the day services by entering your email address into a subscriber box on the website.