Find the top causes of mortality in your area and then try to improve your risk factors for those
Get all of the available recommended vaccinations for diseases that are endemic in your area, including boosters on the recommended schedules; if it's available, always get a seasonal flu shot every year; also try to get vaccinated for pneumonia (reducing your risk of serious complications from other respiratory illnesses); get other people who live in the same household with you to get vaccinated too
If you need complex non-emergency medical care, seek it in a different country with a more advanced medical system
Try to mitigate exposure to major causes of mortality and morbidity (air pollution, contaminated water, contaminated food, violence); try to ensure that you personally live in a building with adequate ventilation, water supply, sanitation, and physical security; if your water quality is questionable, consider filtering and/or treating tap water somehow before using it for bathing or dishwashing (not just drinking)
Don't eat raw or undercooked food of any type that has a history of contamination in your area; don't wash food with water of uncertain quality
If you have any influence with any level of government, try to get them to improve sanitation (sewage, street cleaning, trash collection), air quality, water quality, and food safety inspection procedures in your area
Take exposure precautions when there is an epidemic or outbreak of contagious disease (like right now with COVID-19, but also during future outbreaks of other pathogens) -- hand sanitizer, hand washing, wearing a mask, reducing your physical contact with others, avoiding crowded areas
Find screenings for diseases that increase your risk of death (diabetes, high blood pressure, heart and lung diseases, cancers) and repeat the screenings periodically; consider getting simple devices that would let you monitor indicators for some of these at home (e.g. a blood pressure monitor)
Try to maintain and improve your fitness, nutrition, and mental health
Maintain a good social support system (family, friends, religious communities)
I forgot a couple more related to accidents...
Always wear your seat belt in a car
Always wear a helmet (and other protective clothing as needed) when riding a motorcycle, scooter, or bicycle
Always wear proper protective equipment for work and hobbies, for example gloves and eye protection if using power tools or chemicals
If you live in a dense city with a lot of traffic accidents, consider trying to move to a smaller city, unless that would harm your ability to keep up other healthy habits
Make sure your home and work buildings have working smoke detectors and emergency fire exits; if they're available in your country, also carbon monoxide (CO) detectors, which can prevent poisoning from certain malfunctioning heating equipment
If you use electrical appliances in your bathroom or kitchen, make sure that either the appliances or the wall sockets have GFCI devices, a kind of electrical safety equipment that is mandatory in many countries and that prevents electrocution from some kinds of short circuits due to the presence of water; if you use an electric water heater for your shower, have it inspected for safety by an expert electrician
If your city regularly has seriously bad air quality (like from car traffic, industrial pollution, or wildfires), install an air purifier inside your house (you can most likely make one cheaply from a fan and HEPA filter, if necessary)
