Class size is an important factor in determining which instructional method to use for a particular topic. Perhaps you feel that discussion is important for teaching a particular topic, but you have 60 students in the class. Instead of conducting a whole class discussion, first introduce the topic in a lecture and then break the students into smaller groups where they can discuss the topic. Roam the classroom during these small-group discussions to survey the quality of discussions and to pick up on cues that students may still not be grasping the concept. Then discuss results or problems lecture-style.
If all of your students are at the same skill level, you'll have a wider array of instructional methods to choose from. Cooperative learning, learning centers, discussion and direct instruction work well for homogenous groups of students. When you have a group of students of varied skill levels, however, you may need to choose different instructional methods. For example, if you have a group of younger students with varied reading skills, pair advanced readers with hesitant readers and have them read to each other. Hesitant readers will pick up on reading skills from the advanced readers, and advanced readers will reinforce their good skills as they serve as mentors.
The topic being taught influences the instructional method you choose. Some topics, such as causes of the Civil War, are best taught through lecture or storytelling, while other topics, such as how to count by fives, are best taught through demonstration or the use of math manipulatives. Consider how the topic would be taught most effectively, and don't be afraid to try a new instructional method if it will help your students to grasp that topic.
If you have always used the same instructional methods with your learners, consider trying a new method. Although you may be introducing new material, if you always use the same instructional method, students may tune out. Reinvigorate their interest by using a new instructional method. If you have always used lecture, for example, try using a demonstration or problem-based inquiry. Use instructional methods to keep your students' interest and keep your teaching dynamic.