If you're using any kind of brace with a foam padding to it, take some self-stick bandage, wrap some of it around your calf just tight enough to stay in place without putting any pressure on the tissues, from just below the knee about halfway down the calf, then cut the rest off and save for future use. you should only need about 2 ft of wrapping. then put the brace on. it gives the brace something slightly tacky to stick to, and keeps it from sliding. Skin isn't enough, it just slips, no matter how tight you crank the straps.
trick works for immobilizers or postop braces both.
then you can wear your sweatpants or whatever over your brace. I found that trying to wear clothing UNDER the brace makes it slip even worse, as the fabric against skin is even more slick. As the other poster noted, fabric like jeans is rough, and grabs, so you could get away with wearing a brace over jeans if you had to, but most doctors will tell you that the brace has to be worn next to the skin.
Heck, I've lived in my various braces for so long, I've got sweapants I've modified with a zipper in the one leg just for doctor visits, airports and courthouses just so I don't have to "drop trou" to display my "hardware" to security, doctors, and anyone else that needs to give my leg a visual inspection. I have gotten over being shy.... but man, I am NOT about to drop my pants in a security aisle because somebody with a badge just HAS to visually inspect my brace even though they've given it a thorough "feelie" already and know it's actually a brace!