The potential energy savings from reducing drafts in a home may range from 5% to 30% per year. Here are some tips from energy.gov:

On the outside of your house, inspect all areas where two different building materials meet, including:

All exterior corners
Outdoor water faucets
Where siding and chimneys meet
Areas where the foundation and the bottom of exterior brick or siding meet.

Inside your home, inspect around the following areas for any cracks and gaps that could cause air leaks:
Electrical outlets
Switch plates
Door and window frames
Electrical and gas service entrances
Baseboards
Weather stripping around doors
Fireplace dampers
Attic hatches
Wall- or window-mounted air conditioners.
Cable TV and phone lines
Where dryer vents pass through walls
Vents and fans.