I start the year with the state standards and map how I will apply them throughout the year to best prepare my students for state testing in May, noting the sequence of approximately 2-3 week units in my lesson plan book. Using standards-based assessments, I backward-map from each of the formal assessments, which I administer every two - three weeks (about) during the year. I list the standards that will be covered on the assessment, divide them up in the 10-15 instructional days we will have, with emphasis on the 'power' standards that will have the most questions, and concepts I anticipate will be most challenging. I plan for each two week interval, starting about a week in advance. I then prepare power point presentations, create learning activities to support the concepts, and gather resources I will need - books, photos, examples, rubrics, graphic organizers, sentence frames, vocab cards, etc. I jot down in my lesson book which lessons/activities I will use each day, including bell work, supplies needed, checking for understanding strategies, etc., with extra room in each unit to reteach as needed, or preview concepts from the next unit. So I am never unprepared or attempt to "wing it," which to me is the opposite of good teaching and would be a disservice to my students. Having said that, I also never prepare a written lesson plan unless I am required to, or want to share a lesson with a colleague. I have limited time, which I try to use efficiently and balance with a personal life, and writing down what is already so well-planned cognitively, would, for me, be time-consuming while contributing nothing substantive to my lessons or my students learning. To answer your question another way, veteran teachers are (I hope) not actually 'winging it,' but are working from prior experience with the same lesson that is stored, along with needed resources, in their classroom and/or brain.