That's a common and frustrating problem! When the liner slips down, it makes taking out the trash a messy chore. Luckily, there are a few simple tricks to solve it.
The easiest method is to use binder clips or small spring clips. After putting the bag in, fold the top edge of the liner over the rim of the bin and clip it in place at several points around the circumference. It's cheap and very effective.
For taller, narrow bins, a bungee cord can work wonders. Hook a cord around the entire rim, over the liner, creating tension that holds the bag snugly against the inside walls.
You can also use strong double-sided tape. Apply a few strips around the top outer rim of the bin before putting the liner in. Press the top few inches of the bag firmly onto the tape. Just be aware the adhesive might leave residue.
If you want a dedicated product, look for trash can liner grips or rings. These are plastic or silicone rings that sit on the rim, and you tuck the bag's edges under them to create a secure hold.
Finally, sometimes the bag itself is the issue. Using larger, heavier-duty bags that are a better fit for your specific bin can often prevent slipping on their own. The extra material gives you more to work with at the top.