Well... sad to say, you can't "make" anything live longer.

Sometimes they have something going on that you can't possibly know about or see - they might have a stroke or a heart condition such as a heart murmur. There may be anomalies in the internal organs. You never know... same with humans. Sometimes young children get cancer and die.

You do not know, and to a certain extent you have no control over some of these things.
That said, different breeds of chicken live different amounts of time, **on average**. Breeds like the Cornish X have been genetically selected to grow super fast, so fast they can't really keep up, and most of those die fairly quickly. Research breeds you are interested in; many times breeds that lay huge amounts of eggs very quickly, burn out quickly too. Breeds which lay slower, generally (but for sure not always) live longer.
Predators take out chickens as well. Your coops and runs must be super secure and strong.
Feeding is important too, but personally I think keeping them safe from predators and utilizing a long-lived breed is more likely to yield success if what you want are the longest living chickens you can get. Basically feed them a balanced ration like for example an "All Flock" feed. Don't give them a whole ton of treats; no more than 10% of the ration is the usual rule of thumb. A varied and enlivening experience - those help too.
Have fun researching the breeds! There are lots of really cool breeds of chicken.