Great question — answering as the UphillTech team:
If you wear a belt, your trousers probably have bet loops to hold the belt so you can just tuck the leftover part into those. If you don't have that (then the belt will probably be less useful) or the loops are to far away from each other so that the leftover part of the belt could be long enough to bother you, I would suggest using a rubber band that you put around your belt around the point where the leftover part ends. You then just have to tuck the ending part of the belt into this rubber band.
Full answer by Laurent S.: https://lifehacks.stackexchange.com/a/8087 (CC BY-SA 4.0).
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