Install a water-saving 2.5 gallon per minute showerhead. In the attic and basement, plug the air leaks, and replace and reputty broken window panes. Clean or change air filter monthly on your warm-air heating system during winter and on air conditioning units in the summer. Insulate the first three feet of hot and inlet cold water pipes. Install a compact fluorescent light bulb in the fixture you use the most.
Windows deserve more attention because they outnumber doors.
To reduce this loss, you can either fix up your existing windows or have them replaced with new energy-efficient units. Caulking is your best bet for windows in relatively good shape with cracks thinner than a pencil. There are many caulks to choose from. Adhesion to surfaces and flexibility are key to caulk's effectiveness at stopping leaks. Read labels carefully.