The quick fix
Settings lowered during unoccupied periods will reduce fuel usage, saving money, while providing safety and maintaining comfort during occupied periods. Factors to consider when determining what the correct settings to recommend to your customer should include structural requirements, warm air furnace requirements, and recovery time. You will want to make sure pipes don't freeze and you don't reduce return temperature to dewpoint in the primary heat exchanger. Determine which fuel will be used for recovery and make sure the recovery time is acceptable.

The long term
More effective, long term solutions, often require an up front expense. Having an energy efficient heating system will make a big difference in fuel cost in the long run. Remind the customer that the savings may even make up for the payments to replace the equipment. If new equipment is not an option, keeping their existing equipment maintained and tuned will not only keep energy costs lower, but help prolong the life of the equipment.

What you can't see
Air leaks in both the occupied space and air ducts can be a big factor in energy usage. While ducts behind walls may not be accessible, you can make sure ducts in unconditioned spaces are tightly sealed. Verify that conditioned air is going to the conditioned space and unconditioned air is kept out. A heat recovery, or energy recovery ventilation system, is important for controlling air infiltration and exfiltration reducing heat loss and keeping breathing air healthy.

Customers will be looking to you for your expertise. Keeping up with the industry recommended standards, listening to your customers, sharing your knowledge, and giving them honest, educated recommendations will not only earn their trust, but will help you grow your business.