Writing is like riding a bike--you never really forget how to do it. I expect your writing skills will come back to you once you're back in the college environment. Moreover, a master's program is just as much about developing analytic skills as writing ability. If you already hold a bachelor's and a master's degree in any field and have taken some undergraduate English courses, you are well qualified for a master's program in English.

Here's what I would recommend:

1. Write a Personal Statement. You can kill two birds with one stone by making your M.A. application into a writing refresher course. Most Master of Arts programs in English will require a personal statement (possibly also a writing sample) as part of the application. This can be a perfect exercise for reviving your writing skills. The personal statement is a short piece on a topic you know plenty about--yourself. Write a draft and find a friend or relative whose writing ability you respect to help you remove the rust from your writing skills.
2. Start a daily journal. Even just a paragraph or two reflecting on your day could be enough to get the writing juices flowing again. Like doing weight-lifting, this regular regimen will build your writing strength.
3. Take a class. If you still feel rusty, a college English course will certainly get you back into the swing of things. If you have already taken a lot of humanities courses in undergrad school, you might even consider a graduate course.

Taking a college refresher course is a good option, but not necessary unless your M.A. program specifically requires undergraduate coursework you haven't completed. Talk to the admissions counselor about your academic preparation. If a master's program admits you, you can consider yourself well-prepared. You don't need to be a pro already--that's what you're going to grad school for!