Some of the cheapest ways to reduce the amount of energy used in a building can be achieved by: 

• Orienting the use of the building to maximise solar gain
• Make best use of natural ventilation
• Adopt a good daylight lighting strategy
• Reduce air leaks and droughts
• Increase shading to minimise the need for artificial cooling

For a larger investment you could:

• Increase insulation levels in the walls and roof
• Specify double or triple glazing
• Install wood chip boilers
• Fit a ground source heat pump
• Use a combined heat and power system
• Install wind turbines
• Use photo-voltaic