Seal basement air leaks
Sill plates and rim joists are usually poorly insulated (if at all) and very leaky. So if you have an unfinished basement, grab some silicone or acrylic latex caulk to seal the sill plate. If you simply have fiberglass insulation stuffed against the rim joist, pull it out. Run a bead of caulk between the edge of the sill plate and the top of the foundation wall. Use expanding foam for gaps larger than 1/4 in.
Seal electrical boxes
Electrical boxes that hold switches or outlets are major sources of heat loss. Foam gaskets may not completely seal the boxes, but they’ll help. They’re quick to install—just take off the cover plate, stick the gasket over the box, then put the plate back on.