One of your most important functions as a copy editor is proofreading. As an editor, your command of grammar should be nearly flawless. A relatively simple method of keeping grammar skills developed is to study grammar handbooks. Certain grammar rules can be easy to overlook or to forget, but reviewing a grammar handbook or other reference book, such as a dictionary, from time to time can ensure that your editor's eye stays sharp.
Proofreading skills can also be improved by reading aloud. Reading aloud allows you to "hear" mistakes in a text that you may have missed with your eyes. As you read the work aloud, you may stumble over a certain word or phrase that is hard to understand or that doesn't make sense. You should always be able to recognize anything that makes the text awkward or hard to comprehend, and sometimes reading aloud can help you catch mistakes that you would have otherwise missed.
Good grammar is not the only ingredient of a well-written article. The work should also be concise, clear and engaging. Editing for content, the most subjective of a copy editor's duties, is not an easy talent to build or to practice. A simple exercise to help sharpen that skill is to read voraciously. Read your own work, read newspapers, read online content, read whatever is at your disposal. As you read, you may want to make notes of content that is effective and content that is ineffective. The frequent reading and analysis of fiction and non-fiction on a daily basis can help you to recognize high-quality content when you see it.
Of course, the more you utilize your skills as a copy editor the more you will be able to improve them. The adage "practice makes perfect" applies as much to copy editing as it does to any other talent. If possible, copy edit on a daily basis. Daily exposure to different styles of writing or different genres will allow you to expand your knowledge and skills, which will ultimately help you to better serve the writers for whom you edit. Your ultimate goal should be to help your writer create his or her best possible work, and constantly strive to perfect your own skills as a copy editor will help you reach that goal.