One big mistake many students make when it comes to a thesis is to make it too big or too vague. Then they need to spend hours and days later to refine their subject so that it's manageable. So the one key tip is to make the topic as small as possible.
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To help, you can define:

This helps narrow things down a lot. From there, it's always possible later to make your project bigger if you could fix everything before the deadline. That's a better problem to have than being one month before the end of delivery and thinking: shit, it's still too big.
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Another tip is to be opportunistic. Work with people you already know or that you really want to get to know. Don't make things harder for yourself than what they already are because the goal of a service design project done in a university or school is to learn service design, not to become a king in project management, change management or sales.
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The same goes for a thesis make it as focused as possible by mixing elements. For example, for my thesis, I was interested in using "Humor in social emergency services.". That's super narrow. "Humour for better services" would have been too wide.

Occasionally, I'm lucky enough to coach students from the Master Service Design of the Lucerne School of Art and Design (HSLU) in Switzerland. This question and answer are inspired by past answers I gave during the personal coaching sessions of this program.