Apply caulk around windows and doors, floor seams, vents, and any other small air leaks; use spray foam to fill larger gaps

Use foam weatherstripping to seal larger cracks around doors and windows; add rubber or brush "door sweeps" to door bottoms 

Add outlet sealers (pre-cut foam rectangles) to outlets and switch plates on exterior walls

Use a draft stopper to block air under doors; buy one, sew your own, or roll up old towels or blankets

Install an attic hatch cover to block heat loss around attic doors

Install a vapor barrier (plastic sheets that stop air and moisture leakage) in basements and crawl spaces 

Insulate your attic or living areas above unheated garages; use thick fiberglass rolls, foam blankets, blown-in insulation, or spray foam

Insulate your foundation or basement walls to stop heat loss at ground level; use foam board, spray foam, or other moisture-proof materials

Add insulation to exterior walls to keep heat in; use blown-in or foam insulation for existing walls

Replace old windows and doors with energy-efficient models (look for a high "r-value" and Energy Star certification); don't forget bulkheads and attic hatches

Use storm windows, insulating inserts, honeycomb shades, or even thick curtains and quilts to insulate old windows; volunteer with a Window Dressers team or request custom-made inserts 

Insulate your hot water system to save; add a foam "blanket" to the tank and foam or fiberglass covers on hot water pipes