If your garbage bin doesn't easily fit a standard-size bag, mess ensues. You can fix all that with a simple DIY cardboard insert.
Instructables user goland put together this cardboard insert that makes your bag fit snugly in any size of garbage You'll need a cardboard sheet that is nearly the height of your bin and as wide as the circumference. Goland notes that if the bin will fit inside a cardboard box, it probably has enough cardboard for the project. You'll need to fold the cardboard to fit the can, make a bottom, and some pull handles for your insert, and that's all there is to it. Hit up the Instructable for the step-by-step description.
Hack your trash can [Instructables]