Isolate the main idea together as a class. Assuming that it is a thesis sentence or even a short paragraph, ask students to highlight the main idea with a colored highlighter.
Read the main idea out loud to your students to appeal to your auditory learners.
Ask students to read the main idea to themselves to appeal to the visual learners.
Direct students to take a pencil (not a pen) and write notes and questions about the main idea. Encourage them to write down their observations of what they have read as well as what the main idea causes them to wonder. In this way, you are encouraging active, not passive, reading. Active readers are engaged in what they read and therefore tend to comprehend at a higher level.
Instruct students to write two paragraphs about the main idea -- one that agrees with it and one that disagrees with it. By amplifying the main idea and putting original thoughts in their own words, students should fortify their comprehension skills. After all, you cannot agree or disagree with something you don't understand.
Break students into two groups, debate style, and have them address the relative strengths and weaknesses of the main idea. Encourage a free exchange of ideas and make clear that there are no "right" or "wrong" answers -- only opinions designed to enhance their comprehension of the main idea.
Construct a story pyramid if you are trying to teach comprehension of main ideas that extend beyond a sentence or paragraph. On a separate sheet of paper, have students draw eight lines in descending order of length. In other words, the lines should grow longer with each of the eight lines in the pyramid.
Fill in the pyramid as a class: On the first line, identify the main character; on the second, two words that describe the main character; on the third, three lines that describe the setting; on the fourth, four words that describe the story problem; on the fifth, five words that describe one event in the story; on the sixth, six words that describe a second event; on the seventh, seven words that describe a third event; and on the eighth, eight words that describe a solution to the problem.
Give students time to practice this technique -- recommended by the Columbia Education Center, a consortium of teachers from 14 western states. In time and with practice, students should be able to complete the pyramid on their own.