There is no recipe that will guarantee that your dissertation will receive a top first. But there are certain things that, time and again, lead to a lower than expected dissertation mark and can ruin a degree result.

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Fortunately, they are easily avoided. The most common are:
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Leaving it until the last minute - easily avoided, and one thing that is guaranteed to ruin your last spring/summer of your undergraduate degree as well as (invariably) your degree result.
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Spending too little or too much time on it - spending too little time on your dissertation is often a result of leaving it to the last minute. But it is equally important to be realistic in your planning and not to neglect working on your examined modules.
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Plagiarism - Stupid. Just plain stupid. And even if not intentional (it happens) it can ruin your degree (in some cases, it will end it).