When your garbage bag gets too full, it can create a vacuum that makes it difficult to remove the bag. To counter this effect, tape a tube to the inside of the can. This helps keep air flow going to the bottom of the can so the bag won't get stuck.
While another common solution is to drill a hole in side of the bin, this not only makes it possible for material to leak out, it can also allow pests to get in. It also ruins the bin if you ever want to use it for something besides bagged rubbish. Rather than physically modify your bin, Imgur user samarei suggests taping a solid tube to the inside.
Samarei uses a copper pipe, but you could easily use PVC or any other material that won't compress when your bag gets full.
Trashcan Vent [Imgur]