Consider a lesson or fact that underlies multiple sections of the curriculum. For example, the evolutionary process of prehistoric man is linear. Students will be taught about different stages of man's development, as well as how man developed in different parts of the world. Your big idea statement should emphasize the concept behind this evolutionary development, thus linking each lesson plan.
Tie other disciplines into your big idea statement. If students have learned about the theory of evolution, this knowledge is especially appropriate to emphasize man's development in prehistory. Another idea statement might focus on sociological aspects by relating prehistoric civilizations to more recent civilizations that students have studied.
Write the big idea statements concisely. The statement should be conceptually based rather than detailed. One example of a big idea statement is, "Man's development of agricultural techniques made way for complex societies that engaged in trade." This demonstrates the evolutionary process of society in prehistory and links it to the economic concept of trade.
Have your students come up with their own big idea statements. Though your big idea statements help students better grasp your teachings, creating their own statements demonstrates that they are able to apply their knowledge. Encourage students to think conceptually to find the underlying ideas in your prehistory curriculum.