From some scrap fabric we now make two ribbons to fix the pants while wearing.
For some light weight fabric like silk or similar, you could also think about fixing 4 ribbons on each end and then make small knots while wearing.
For heavier fabric like linen, I used a piece of Velcro fastener. Add the rough part to the pants and the soft to the ribbon. In my case, I added an additional belt loop at the edge of the fabric to act as a support for the ribbon.
The ribbons must be at least as long as the gap of 1/3 of your waist circumference plus some 10 centimeters more to fix them in the hem and on the other side.
My hint: Make them much longer and later adjust the length that is sewn in the hem. Make one good looking end that is outside and trim the end that goes inside the upper hem of the pants.