Write. It's that simple. Even when you don't want to. Even when you don't think you have anything to write. Even if all you write is a page full of "blah blah blah." Just stay in the habit of doing it so that when you do have something it won't feel so foreign and won't take so long to get into gear. If I've gone too long without writing and I have something spark that interest it takes me a long time to get up to cruising altitude. Athletes have to train to stay in top form and I think artists and writers are no different.