Start to answer this question by first understanding how much insulation you already have.  The next step is to consider how much savings can be attained through various increases in insulation levels of your home.  Many older homes have less insulation than new homes built today. A good home insulation contractor will include an insulation check as a routine part of an energy audit. For information about home energy audits, call your local utility company. An energy audit of your house will identify the amount of insulation you have and need, and will likely recommend other improvements as well.
If you don't have an insulation contractor inspect your home, you'll need to look for insulation in several places.

• In unfinished attic spaces, insulate between and over the floor joists to seal off living spaces below.
• Attic access door
• All exterior walls, including walls between living spaces and unheated garages.
• Foundation walls above ground level
• Foundation walls in heated basements, full wall either interior or exterior. 
• Floors above cold spaces, such as vented crawl spaces and unheated garages
• Any portion of the floor in a room that is cantilevered beyond the exterior wall
• Slab floors built directly on the ground
• Unvented crawl spaces
• Extend insulation into joist space to reduce air flows.

Storm windows, caulk and seal around all windows and doors as a final step.