First look at and rank how you are using energy
1) Transportation,
2) Lighting,
3) Heating,
4) Cooling,
Then try for reductions in the most heavily used areas first. Your local energy supply company should be able to help you out with an energy audit of your home or business. Look carefully at your energy bills to see where savings would be most useful.
I can't tell whether you are asking this for a company or a residence. Check out the talk by the building managers at Adobe systems that you can download and view at
http://www.parc.com/events/forum/archive.php
George Denise, CFM, CPM, FMA, RPA, General Manager, Cushman & Wakefield February 1, 2007
Electricity is generated least efficiently when the most is needed - such as mid summer afternoons when everyone is running their air-conditioning.
Make sure that you have minimized the number of inefficient incandescents used for lighting.
Look for air leaks that would increase your heating / air conditioning and increase the insulation under your floor (check out reflectix or equivalent which can be stapled to joists from a crawl space) or above the ceiling - add fiberglass.
Also you will really notice the difference if your hot water pipes are not insulated - just apply the foam pipe insulation where you can.