Fill gaps with foam
Electrical outlets and switches on exterior walls can leak a lot of cold outside air into the house. Add up all the outlets in the average house and you can have some serious heat loss—which makes it worth spending 10 minutes per outlet plugging the holes.
Before you start, flip the circuit breaker off and use a noncontact voltage tester to ensure there's no power. Remove the cover plate. If the gap between the electrical box and the drywall is less than 1/4 in., fill it with acrylic latex caulk. If the gap is bigger and lopsided, use foam sealant that’s formulated for use around doors and window framing. The minimally expanding foam won’t drip down your walls. After the foam dries, cut away any that protrudes, add a foam gasket (to reduce drafts through the box) and replace the cover plate. Do the same around register openings on the inside of exterior walls.