This was one of the first questions I ever asked the forum. I have the same problem -- always have, and probably always will. I think it's partly my shape, but it seems that a bit of sliding is not uncommon.
This is what I think is going on. Most women have some flesh around their midsections. Even a small amount of flesh will result in muffin top if the pants are too tight. Ergo, the pants can't be too tight; they must be *just* tight enough. But, most jeans contain some percentage of stretch fibers, and that causes a bit of fabric flex and inevitable downslide.
This problem has been bugging me for years. I've asked all my friends, and the only ones who do not have the problem of denim jean slide are the ones who have VERY lean, flat bellies. (Angie fits this description, as it happens). It doesn't seem to matter whether their hips and thighs are lean or fleshy; it's the midriff that counts. A woman with no extra flesh on her midriff can wear her jeans waistband as tight as it needs to be to prevent downslide *without* causing flesh spillage. I've also noticed that my eight-year-old daughter, who, like many prepubescent girls, has zero fat around her middle, does not have the problem of jean downslide either.
Anyway, that's my best guess as far as explaining why jean downslide happens. As for solving it, there is lots of good advice here. For me personally, the things that help the most are:
a) get the waist and seat taken in so the jeans fit as perfectly as possible
b) wear a thin, flat belt (I hear invisibelts are good, although I don't have one)
c) don't get jeans with too much stretch in them (my jeggings, while very comfortable, do require a belt to make up for the stretchiness).
Ironkurtin: when I have the waist and seat of the jeans taken in, they do it by opening up the back seam, removing some fabric, and sewing it back together. There ends up being a seam all the way through the waistband but it is concealed by the back belt loop.