Most Helpful Guy
That will be as it seems girls jeans are usually either low riders or bloody high waist which is unattractive on most people so you end up with jeans that sit at underwear level when standing. While you can look for ones with a better cut and more ass room so it doesn't slip down straight away you would find it easier getting longer fitting tops.
I get a similar problem when try wearing mens jeans as got a big bum which means sit down and jeans slide down and show lots of underwear, so I just live in military surplus camos as they are cheap comfy practical and only thing that has a cut that fits me well.
Most Helpful Girl
You could always try high waisted pants. Plus, you won't need to wear a belt if your jeans are high waisted because the part that goes around your stomach likely won't slide down over your hips too easily. If you do prefer lower rise jeans try tightening your belt and buying the pants a size too big. Most leggings or tights have a fairly high waist too so it shouldn't be a problem when you're wearing them but if it is try to find a pair that fits really snugly. Longer shirts are always an option depending of the look you're going for. I'm positive sweats will work if all else fails for the days you feel kinda lazy. I think that a g-string would show less as well because its less fabric to cover and its closer to the top of your butt than just a thong.
I've never had this problem personally just thought up a few ways that might help to cover it better. Hope I helped.