Hmm. I’m thinking there could be two possible issues. First, you might have the wrong size! If you have a small waist and narrow hips, your jeans might just be sliding down as you walk and move around. In that case, you could size down, but it might be good to look for jeans that are cut more narrowly too.
Another thing I’m suspecting is that the jeans you are buying don’t have quite the right material to accommodate the stretch and then return to their normal size. All jeans stretch to some extent as you wear them, but the higher the cotton content, the less likely they are bounce back from stretching. Often, a high cotton count jean will stretch…and then remain in the shape they’ve stretched to. Once they’re washed, they’ll do it again, though they’ll probably feel like they fit great IMMEDIATELY after washing. On the other hand, washing tends to break down denim and a lot of DENIM EXPERTS say you shouldn’t wash them at all. I’ve seen advice that says to eschew washing, put them in the freezer for a while and then turn them inside out for a bit in the dryer to reset the stretch. (I have never done this.)
So, it’s kind of a sick balancing act: The more synthetic materials you have in your denim, the more likely the denim is to bounce back from a stretch and maintain its shape. But, the more synthetic materials in the denim, the more stretchy they are to begin with and they could continue to stretch all the way up to a bigger size.
To try and figure out if the material in your denim is the problem, I checked the content of AE’s skinny jeans (this pair, $25). They are 71% Cotton, 14% Polyester, 14% Rayon, 1% Elastane. This is actually a pretty good denim-to-synthetic makeup if you want a lot of stretch (i.e., they’re probably very comfortable), but this might be TOO much stretch, especially if you happen to be between sizes. You could try to size down. Or, you could look for a jean that has a higher cotton content so you get more denim rigidity—they’ll feel less stretchy, but if you get the right size, they won’t stretch out and sag. AE’s high-rise skinny jeans ($40) are 98% cotton, 2% Spandex. That’s probably a better option for you. They’ll feel a bit more “stiff” than the other pair, but they’ve got enough give (thanks to the Spandex) to be comfortable and bounce back to their original shape. If you want to try a jean that’s between the two, Gap’s 1969 Always Skinny jeans ($70) are 87% Cotton, 12% Polyester, 1% Spandex. It has more cotton but less synthetic than the original pair I discussed above and less Spandex than the high-rise pair from AE, so they might be a happy medium for you. Another idea: I have several pairs of Old Navy’s The Rockstar jeans ($25) and they haven’t stretched out at all. They’re stretchy enough to be quite comfortable, but they’ve retained their shape so well. I get the smallest size I can fit into without turning myself into a human sausage. They’re 70% cotton, 28% polyester, 2% Spandex. That high synthetic count is what helps them retain their shape and the Spandex gives a comfortable stretch. I’ve also had great luck with H&M’s treggings. Once again, I buy the smallest size I can and they hold their shape and fit really well. Because they just have an elastic waistband, you don’t have the weight of a zipper and pockets dragging the crotch down either. They’re $18. The ones I have are 3% Spandex, 27% polyester, 70% cotton.
Worth noting: I’ve read that the best high-end jeans (pairs around $200) often have a higher cotton count with a 2% synthetic material, usually polyester or nylon. That synthetic allows for a fantastic close fit and the high cotton count gives you structure that won’t stretch out or sag. See the AE high-rise skinny jeans—they have an identical fabric count to this $181 J.Brand pair.
Now, if you’ve tried all this and you’re still having issues, my last resort idea is to buy a pair that’s the next size down—even if they’re a tad uncomfortably snug—and wet stretch the areas that are too tight. This works better on jeans with a higher cotton count. Simply wet the too-tight areas (like the waistband) and then wear them around the house. Sit, stand, do lunges, stick a book between your waistband to help stretch them further. Once they are dry, the fit should more comfortable. You can repeat this as needed.