Because I was reading such vastly different styles, for example, a children's book followed by "Little Women," I saw my writing style shift momentarily based on what I was reading.
Now, I'm not the biggest fan of "writing exercises," mainly because I like to stay in my own world, and most exercises are writing prompts that throw me somewhere else. This, however, was perfect for me because I could simply write in my own world and watch my style change.
What are you reading right now? The next time you decide to carry on reading whatever it is, be prepared to try writing immediately afterward. For maximum effect, you should be writing as soon as you put the book down.
You don't have to try to copy the author's style. Just see where the book takes your writing, and let it happen naturally. You could be writing about anything. Continue your own story, or, if you like prompts and need a boost to get going, look up a quick prompt. For best results, find something to write about before you start reading so the transition is as smooth as possible.
Save your work. Put the title/author of the book you were reading above your writing segment. Your writing can be short or long, however much you want to do. Then, come back to it whenever you read a new book. Try it several different times, with different genres/styles. The more you try out different things, the more you'll see an effect.
You might feel like your "copying" other authors, but this will help you develop your own writing style. You can check back over it and see what elements came from other authors, and what is consistent throughout (that's yours!). This will also help you determine what you like, what you don't, and what works best for your story. Plus, it's just entertaining, at least, I find it to be.
But in order to do this exercise, you have to be following your golden rules! So get to reading, and then write!