A little bit of caulk goes a long way around your windows and doors. There are special thermal imaging cameras that can illustrate drafts in real-time, but you don't need to get that fancy. Go to your favorite home improvement store and buy a few tubes of door and window caulk. Next go around the entire exterior envelope of your home and caulk around all of your windows and exterior doors. You can also caulk around vents, electrical wires and utility boxes etc.
Attics and basements are often overlooked, study the diagram below and caulk or insulate around any cavity that can cause air leakage to occur.
When your home is sealed and insulated properly, you will spend less money on heating and cooling costs. And don't forget that you can do most of the work yourself with a minimal investment. Conduct an entire house audit of your home with a notebook and a pencil. Go through each room and identify the areas where you think air leakage can occur. This includes any opening into the exterior, basement, or attic. Keep in mind that older homes will require more insulation and caulking then newer homes.
Common places to seal around:
Click the picture to enlarge it
Ducts (get silver duct tape and seal the seams of your duct work)
Electrical Wires and Components (cable wire from the exterior coming into your house)
Once you complete these simple steps, your home will be less expensive to heat and cool. Filling little gaps doesn't seem like much, but once you complete the process you will be able to see the difference in your energy costs!