Hardware stores carry foam gaskets that install behind the cover plates. Very inexpensive, but pretty effective and easy to install. It can also help to caulk around the outlet (behind the cover plate) - that little gap between the outlet box and the wallboard. More complex, is to pull the outlet receptacles out and caulk the openings in the back of the outlet box, including where the wires enter the box - you must shut off the power to do this! Be real careful working around outlets.