low quality jeans. Good quality is recognized by them not stretching during use, staying solid from rough use, and maintaining the color. Then again, very few people need top quality jeans unless they want the pair to last more or less a lifetime.
Just use a belt or something? I'm a skinny bastart who can't avoid sagging at the START of the day, so i have to use belts. do the same
Try a different brand of jeans. American egale jeans have a tendency to do that - my experience - so I switched to a different brand and I haven't had the problem since... Maybe you should switch from the jean brand you are wearing. Once you find that perfect pair it will feel like life is complete lol.