Amanda, Tania, and Andy: Your jeans stretch out because they are not 100% cotton and have some sort of elastic/spandex/whatever in them that relaxes as you wear them from motion and body heat, and because woven fabrics generally tend to stretch when you put pressure on them in one place. Also, you are curvy ladies, at least enough so to experience otherwise perfectly fitted jeans that slide down off your butt even when wearing a belt, and this is because the jeans are not as curvy as you. If you are wearing [american] odd sizes, these are cut for teens. Try (women's) even sizes to see if that works better, because it did for me. Otherwise, you can cut your waistband at the sides or the back and take it in, but I would recommend asking someone else to do this if you are not an experienced sewing type because it is hard to work with so many layers of denim and get a good clean dart.
To be fair, I do not know how to avoid elephant knees 100% of times, but I pull my jeans legs up a little when I sit or whatever like I have seen my granddad do to his trousers to put the bendy part of my leg that takes up more space in an area that, well, has more space in my pants leg. Bonus is showing off my beast socks.