Check that the conclusion poses something new and interesting. A friend or colleague may be able to assist you with this process by providing feedback.
Avoid repeating information. Summarize the dissertation's main points (bullet points might be good for this). The purpose of the conclusion is to demonstrate what you have learned from the research.
Demonstrate your knowledge by referring to examples of the work and research you undertook in the dissertation. Statements such as "It was established from the experiment on chemical analysis in section 6.6 that ..." are useful. They provide recall for the reader about the topic, the evidence presented and where to find the information.
Provide your own insight and opinion. The reader may not necessarily agree with an opinion you provide in a conclusion but will respect it if your interpretation seems to be reasonable and makes sense. Sometimes an interpretation is neither correct nor incorrect, as it is simply a subjective (your own opinion) interpretation. Problems only arise when you are asked to provide an objective (not affected by your own opinion) interpretation, and you provide it subjectively.
Avoid emotive statements in the conclusion. A dissertation is supposed to be an analytical paper, not a speech on a specific subject. This may be difficult to do sometimes but is a requirement of any dissertation.