It's a brilliant solution! I don't know why I didn't think of it years ago. The elastic or whatever has gone in at least two pairs of my tracksuit pants, but it has always seemed a shame to throw them out. So I haven't, which means they keep slipping down whenever I wear them.
Anyway, if you are faced with this problem, what you do is grab enough of the waistband in your hand to make it tight around your middle.
Then you get your average bit of kitchen twine, maybe about 30 cm, less if you're thrifty and efficient, and tie it in a loop around the base of the bunch that you have in your hand. Just a normal knot that you use for tying your shoelaces, nothing fancy, and your tracky dackies will stay up for the entire evening and still be in place when you wake up in the morning.