That's a classic urban living headache! A slippery, round kitchen or public trash can letting the bag collapse in is so frustrating. The best way to secure it is surprisingly simple and requires no special tools.
First, ensure the bag is fully opened and draped evenly over the can's rim. Then, take the classic "ponytail" or "bun" approach. Gather the excess plastic at the top of the bag into a loose bundle. Twist this bundle once, then tuck the entire twisted part back down inside the can, between the bag's liner and the can's inner wall. This creates friction and locks the bag in place. For extra security on taller cans, use a large rubber band or a short bungee cord looped around the rim over the bag. Another pro tip: if your can is very deep, place a lightweight, empty cardboard box or a few crumpled paper bags at the bottom first. This takes up space, prevents overfilling, and gives the liner more surface to grip onto, stopping that annoying slide to the bottom.