You mean your trousers fall down? How are you using the stp? Like, do you completely unbutton, unzip, and undo your belt? Or just unzip? Or something else?
My pants would fall if I unzipped, unbuttoned, and undid my belt. I realized after a while that it was kind of silly to do all of that. So I kept my belt on and just unzipped/unbuttoned. Much easier and the pants stayed up. But I suppose if you don't wear a belt or have belt loops at all, that could be difficult.