That's a great question! A slipping or falling liner bag is one of the most annoying little problems in urban living. For a large public or residential trash can, here are the most effective and practical ways to secure the liner.
First, start with the right bag. Use a liner that's appropriately sized—it should be large enough to have ample overhang over the rim. A bag that's too small will never stay put. Consider using drawstring bags, as the loops can sometimes be hooked under the can's handles or lid.
For a simple, no-cost fix, try the "double wrap." Before inserting the bag, wipe the outer rim of the can dry and clean. Then, pull the top of the bag over the rim and fold it outward, creating a wide cuff. This increases friction and gives the bag more grip on the can's surface.
If you need a more secure hold, use external clips. Large binder clips or specialized trash can clips from a hardware store are perfect. Simply clip them over the bag's edge and the can's rim at several points around the circumference. They're inexpensive and incredibly effective.
Another clever hack is to use adhesive hooks. Place two or three small, sturdy adhesive hooks on the *outside* of the trash can, just below the rim. After putting the liner in, hook a section of the bag's overhang onto each hook. This distributes the weight and prevents total collapse.
For heavy-duty situations, a bungee cord or heavy-duty rubber band can be a lifesaver. Place it around the entire circumference of the can, over the secured bag liner. This method is excellent for windy outdoor areas or cans that get very heavy.
The key is combining friction, mechanical hold, and the right materials. A quick wipe of the rim, a good cuff, and a couple of clips will solve 99% of slipping problems and make taking the trash out much less of a hassle.