Get an adult grammar workbook. Adult grammar workbooks review basic and advanced grammar rules, depending on the level you buy, and usually feature small lessons with exercises. Most adult grammar workbooks also feature quizzes and review tests at the end of the book. You can purchase adult grammar books at your local bookstore.
Write by hand on a regular basis. You, like the majority of the modern world, have probably become dependent on your computer's automatic spelling and grammar check. Writing by hand forces you to reference the grammar and spelling skills you developed as a student. Use a typewriter if you are uncomfortable writing by hand.
Use writing prompts. Fiction writers often use writing prompts to fuel creativity and spark imagination, but any writer can benefit from using these tools. Writing prompts generally consist of one phrase or sentence that you use as a starting point for a narrative or story. Try a writing prompt to work on your written voice and style even if you are not a fiction writer.
Outline before your write. Outlining gives your writing structure and flow, while allowing you to organize your thoughts. Review your outline to see if your paragraphs are in the best order to make your point.
Take a continuing education writing class. Most colleges and universities offer adult education classes in various writing areas such as creative writing, fiction and professional writing. With a writing class, you have the benefit of a trained professor and group critique.
Write outside your comfort zone by doing writing exercises. Writing exercises are more restricted than writing prompts. In writing exercises, you write by certain guidelines that range from a predetermined setting and characters to restricting to certain words and tenses. Switching your style of writing not only improves your writing in general, but can also improve your grammar. Give yourself exercises such as writing a narrative entirely in the second person or future perfect. When you place grammatical restrictions on your writing, you force yourself to follow grammar rules while stimulating creativity.