When teaching my students about essay writing, they often complained about conclusions. Their conclusions were bland and often pointless, and the students knew it. What they didn't know, however, was what to do to fix it.
The key to writing a good conclusion is to make sure that you know what the point of a conclusion is. We often teach students that a conclusion is "an upside down" introduction. For example, I might tell students to start their essay by introducing the topic, indicating the text that they will be discussing, and finally narrowing in with a specific thesis statement. If I tell them to write an "upside down introduction" as their conclusion, that means that I want them to restate their thesis, revisit their textual examples, and go back to discussing the topic of the essay. Now what does this mean to a student? I don't believe that it tells them anything - why would I tell students to write something that they already wrote?- and this is why they write weak conclusions.
In my instructions to my students, I tell my students that the point of the conclusion is to tell the reader why everything that they just wrote about matters. English teachers love to use the phrase "so what," and it is very applicable when talking about conclusions. I have had several students tell me that they don't think that what they are writing about matters at all. After all, this is not an activity that they would partake in if they weren't forced to do it in class. This is when we discuss themes that are important to all humans. Take your thesis and ask why this is matters in relation to the greater human experience. What themes are highlighted? What have you said about those themes? Why does this matter? This is what you want to discuss in your conclusion.
Food for thought... how do you help your students to write stronger conclusions?