One of our strategies that many teachers love and often use only takes a minute or so to share with kids. At the end of the school day (or class period) ask children "What is something good that has happened for you today?" Once they have had a chance to think about that and one or a handful have shared their thoughts, ask them "What is something good that might happen tomorrow?" The second question gets them planning and scanning for something good about school. For children who can't think of something, help them with the thinking and language of the positive.
Many parents who have attended our workshops have chosen to use this strategy at dinnertime or bedtime!