Keep good records of your pond. A chronological log of chemical test results, treatments, maintenance actions, water change outs, and even addition or removal of fish can help determine the cause (and required treatment) of a future problem. I keep mine in a computer file but just a simple notebook is all that is needed.
Keep chemical test apparatus clean. Scrub out the test vials periodically. Just rinsing with pond water doesn't get out all the residue buildup. Whenever you buy a test kit, write the purchase date on it. If not otherwise stated on the test kit, replace any liquid based test kits every year. Replace the sealed packet dry powder and pill based kits every two years.
Be careful about anything that you put in your pond! Know your pond capacity and carefully calculate and measure dosages. Know what you are treating for; it is usually better not to treat at all than to dump in something because you think there might be a problem. Second only to ammonia poisoning, more Koi have died from improper treatment with medicines and chemicals than for any other reason.
Many times, the first indication of a problem can be detected by simply watching the behavior of the fish. Changes to their normal activities means it is time to get out the test kits. This is the best part of having the pond anyway so spend some quality time with your Koi and get to know them.