To any other people blessed with a big bum and comparatively small waist (in the opinion of the clothing industry, anyway), I've found a way to help stop your trousers from falling down.
Yes, one can wear a belt, but there are never quite enough belt loops, leaving you with a belt against your skin and the trouser waistband bulging out on one or both sides at the back, instead of being kept in.
Anyway, yeah. Add an extra button, an inch or so further in-band from the original button. Ta-da!