I don't know about you, but I almost always buy the cheap trash bags without the bells and whistles...OK, without the drawstring or another feature that might help the trash bag stay in place around the top of the kitchen trash can. The thing I have found to help keep the trash bag from falling in (since these cheaper bags also tend to be shorter) is to keep a diaper wipes box in the bottom of the trash can.
The diaper wipes box supports the bottom of the (shorter) bag and also comes in handy as a place to store trash bags so they are conveniently placed and kept clean. I would recommend using a diaper wipes box with a solid lid (unlike the one in our trash can) so the clean bags would stay clean even if the trash bag in use should leak.
An added bonus? Since the box is supporting the bottom of the trash bag, it helps improve the accuracy of children who are being trained to take out the trash and haven't quite gotten the hang of securing all four corners. ;)
Want more WFMW posts on kitchen topics? Check out an easy way to keep up with leftovers/keeping recipes in a notebook and chopping onions without the tears!