I like to be prepared as much as possible for upcoming unit studies. A few weeks before the next month comes along, I think about any new activities as well as older ones and get started on prepping for those now. Today I am thinking about next month and know that the next continent we will talk about is Africa. I also know the season will be changing and need to plan for that.
As an example, for a snowflake art project, I need to get some of those cut edged craft sticks and glue them together for the snowflakes. I also need to cut a magnet strips for the back of each snowflake. You can bring these supplies into the classroom and have the older children do this as well. Then they are complete and ready in the cupboard for the day you decide to talk about snowflakes.
In thinking about the next continent, I need to have any copies ready of that continent as work choices for the children. I need to gather supplies for the continent table and any materials for the shelf such as kinds of animals and plants common to that continent. I need to place the books I will need on hold at the local library and search out new art ideas and prepare for those. There is always so much to do in the classroom and anything you can do well in advance is very helpful, as it should never take you away from your time with the children.