Looking for something to do right now? I just painted my bedroom, so I checked the electric sockets to see if I felt a draft. These holes let in outdoor air, particularly in older homes. Outlets and light switches are usually ignored when it comes to insulation, but can be a source of heat loss.
Foam outlet covers block the air trying to get in, and take minutes to install if you have a screwdriver- they go under the plastic cover you already have. Also, you really only need them on external walls. You might not see immediate savings on your power bill, but little easy steps like this eventually add up!