A loose outlet pushes in every time you insert a plug. Often it happens because the cutout around the outlet box is too big. When the drywall is taped, gaps around the electrical boxes are filled with mud, which supports the outlet ears. In a heavily used outlet, this mud breaks loose, leaving the outlet ears unsupported. Eventually the cover plate cracks. A scrap of 12- or 14-gauge electrical wire and a few common tools are all you'll need to lock that outlet down tight.
Start by shutting off the power to the outlet (Photo 1). Photos 2 and 3 show how to convert scrap electrical wire into a coiled spacer, which will bridge the gap between the outlet and the electrical box. Once the spacer is completed, install it between the outlet and electrical box as shown in Photo 4. Note: If the face of the box isn't flush with a wood or combustible-material wall, or the box is more than 1/4 in. behind a drywall wall, you must add a box extender.
Shut off power to the outlet at the main panel, then check the wires with a voltage tester to make sure.