Are we talking about jeans? If so, you can cut a slit on the inside of the waistband just behind where the side seams meet at each side. Insert waistband elastic that is about three inches shorter than you measure around the back, (side seam to side seam). Using a safety pin or bodkin, slide the elastic in one side until the elastic is all inside except for about 1/4". Using thread to match jeans, zigzag heavily to keep elastic in place and close the slit. Pull the rest of the elastic to the slit on the other side and pin with 1/4" showing repeat zigzag on the second side, remove the pin. Now your pants will stay up until you find the new you-size and begin to buy smaller pants.
If we are talking slacks, you can put small darts in the waistband extending down at least an inch from the waistband seam in several places to keep them up, and keep the pockets to use.
I use to weigh over 500 lbs so I had to figure out how to reduce my clothing sizes, and not go broke. Congratulations on you weight loss. I am proud of your accomplishment. I know how hard it was for me.