They say cliches exist for a reason - because they're TRUE. That old saying of writing every day is no exception. I find that if I don't work those creative muscles, they atrophy. Like any other muscle in the body, if we 'let it go,' we forget how to use it. Even if you can't muster up ideas for your current project, engage in a different writing exercise. If you only write several sentences, it can sometimes spark more to follow. Before you know it, your words are flowing across the page...or computer screen...whichever method you prefer.
On day at work, as I sat in my car at lunchtime, I saw a beautiful butterfly flit across my path. I wrote down a few things about the experience...how wonderous nature is, noting the different colors in its wings, and how it floated through the air. Once I was done, I'd realized I had another IDEA for a portion of my current manuscript.
I'm the type of person that's too easily distracted. If I allow myself to miss one day of writing, a month will then pass, and I'm looking at a blank computer screen...no new ideas...just stuck in my story. That gets me nowhere if my dream is to be published. Books don't write themselves. Even famous authors have to sit down and slog through a manuscript the same way the rest of us do. They're no exception, and I've read that even THEY suffer with writer's block from time to time.
I've become more observant....that truck with the squeaky trailer that passes me by in traffic...the sound of the waterfall in a building's lobby...all become a sentence, and in no time, a paragraph.
Even if you're not a writer - notice the world around you, and it becomes a more interesting place.