Younger students often find it easier to identify the main idea in nonfictional pieces compared to fictional stories. Teachers can ask their students to recall and retell previous experiences or stories they had heard and identify the main idea. In case of stories, most children's authors try to convey a message or a big idea to their young readers through their writing. Children are usually able to identify the morals embedded inside fables. While retelling stories, a child is automatically forced to select and recount the more important details and the central message. Teachers can ask about missing details to point out their mistakes.
When your students read and comprehend a passage in the classroom, encourage them to think about and determine what the central message of the piece might be and ask them to identify the supporting details as well. Ask them how they located the topic sentence, where they found the topic sentence, which sentences contain the supporting information and so on. It will make a student's task much easier if he is encouraged to draw a central message chart while reading the piece. Such a chart helps students internalize and organize the bits and pieces of information they come across while reading a comprehension passage.
Teachers can ask their students to overview or skim through a comprehension passage before reading it in detail. Teach them to observe the subheadings and heading of the passage as well as the text length, show them which portions they should read, which parts they should ignore and which trivia they should skip. Instruct them to focus their undivided attention on certain very critical information and test them on what they have comprehended from the passage. This way, students can quickly learn to identify the main idea and the supporting details in a piece of writing. But this method has to be perfected through relentless practice.
Teachers can read a paragraph from a book, magazine or newspaper aloud to their students and ask them to remember the details while listening. Then you can ask them post listening comprehension questions on the purpose or goal of that paragraph. In this case, students have to be taught to listen properly to the whole idea of the paragraph or they won't understand the purpose of listening and won't be able to identify the fact. However, presenting questions beforehand defeats this particular method.