Have your septic tank pumped out every three to five years. Don't wait for bad smells or slow drainage. Septic pumping and servicing companies pump out tanks and wash them with water, then they inspect them for cracks or damage. Some old systems have fiberglass or even brick tanks, which may eventually have to be replaced with concrete. Cracks in concrete tanks can usually be repaired.
Check the distribution box. This is a device into which liquid effluent flows from the tank and is distributed to the leach lines. Sometimes old distribution boxes have valves where one can manually direct the effluent flow through one or the other leach line. If your drain field isn't working efficiently once the tank has been pumped, and you have this option, try switching the flow to another line and reducing the amount of daily water usage.
Excavate your drain field as a last resort. If you have water lying on top of the leach field, your soil may have become hardened. In time, salts build up and form a hard crust on top of the soil, which prevents or slows the effluent from filtering back into the soil efficiently. Alternatively, you could have broken or crushed pipes, or they may be clogged by roots.
Contact your local health department. You may have to get a permit to do any work on your drain field. Replacing broken or blocked pipes, cutting away any invasive roots and re-working the soil to make it more friable are all options for improving the efficiency of an old septic system. If you're handy you can do the work yourself, but your local authority may require a licensed septic engineer to handle it.