The short answer
Drive it far enough to charge properly, keep the terminals clean, and use a trickle charger if it sits unused. Short journeys are what kill batteries.
- Short journeys do not recharge a battery. Starting the car uses a substantial charge and twenty minutes of driving may not replace it.
- Take it for a longer run every couple of weeks if you normally do short trips.
- Keep the terminals clean and tight. White powder on a terminal is corrosion and should be cleaned off with the battery disconnected.
- Use a trickle charger or conditioner if the car sits for weeks.
- Turn everything off before starting, and do not sit with the ignition on and the radio playing.
What will ruin it
Most batteries fail in the first cold snap, because cold reduces their capacity while the engine needs more power to turn over. A battery that has been slow to start all autumn will fail in December, not in June.
Why it works
A lead-acid battery degrades when left partially discharged, a process called sulphation. Repeated short journeys leave it in exactly that state permanently, which is why urban cars get through batteries faster.
Gas work requires Gas Safe registration by law and most electrical work is notifiable. Where a tip says to call somebody, that is the advice. Full disclaimer.