The question about is a relatively simple answer. To live a healthy life you need a few basics (listed in order of importance):
The reason why I say "order of importance" is based on how long you would last without each of those elements. So let's look at those precious elements again and see how long a person can expect to live without them.
1) Air (about 4 minutes)
2) Water (approx. 5 to 7 days)
3) Sleep (about 11 days)
4) Food (approx. 3 weeks)
5) Exercise (maybe 25 years)
So why do I bring up this, somewhat, obvious fact? Because many of you don't really take this very seriously. Many of you think cigarettes are air, donuts are food, soda is water, and exercise is walking from the car to the liquor store. The other day, I saw a person who was wheezing and coughing in between puffs on his cigarette. From what I could tell, the only exercize he was getting was muscular contractions he was putting on his chest as he was coughing. We all have a certain amount of choices in life and it would appear that some of the choices that person has made is keeping him on a miserable path.
DNA is an amazing thing. It's the stuff that's primarily responsible for life on Earth (including your life and mine). It's a grand gift from our Universe to free us from the rock and soil. For a few precious years, it allows us to have this wondrous experience that we call life. Open your eyes and see all of the strange things around you right now that are not rocks and dirt and you'll be taking a moment to experience the greatest gift in our Universe.
Which brings us to the point here. What does our DNA really need to have a healthy, long (long as possible) life? Let's take each of the elements listed above and go through them. Like I said, "It's a relatively simple answer."
Air - If you've been smoking (smoking anything) for years then you haven't been breathing enough air lately. Our lungs were only designed to breathe Air (78% Nitrogen, 21% Oxygen, 1% other gasses). When you breathe anything other than air then you're probably hurting your body. Just figure that the tar from the smoke (that a smoker breathes into their lungs) has attached itself to the inside of the lungs. The lungs are now not able to take enough oxygen because they're clogged with tar. Oxygen (amongst other things) is needed for the heart to work properly and the brain to think properly. No wonder a smoker can't think straight. They're not getting enough oxygen to their brain. If you do this for years, the injury to your lungs, blood, and other organs may be more than your body can handle. First simple rule of health: Just breathe air. This rule goes for "Air Freshers" too. You think that spary from the can is actually making the air better? WRONG! Read the label. It's probably poisoning you.
Water - So you're thirsty and you need something to drink. Soda? Well, soda does have water, but it also has roughly the same amount to sugar as an entire pack of cookies. Yes, that's a true statement. I'll say it again. One can of soda has nearly the same amount of sugar in it as an entire pack of cookies. And you were thinking of having a second soda?? You're body was not really designed for that much sugar intake. OK, so maybe you'll have a diet soda instead of a sugary soda. In this case, you are still not drinking a healthy beverage. Second simple rule of health: Hydrate yourself primarily with plain water (Should be, at least, 80% of your daily liquid intake).
Sleep - Sleep is appearantly underrated. Many people (including myself) don't really get enough sleep. We work all day, we hang out with our friends after work, we play video games until we find the magic jewel (at 3 in the morning), and then we get up at 6am to start our new day. The body needs sleep. Study after study shows the important function of sleep. It allows your body to make cetain internal repairs. It allows your nervous system to "reset itself" for another waking cycle. Your heart can rest so it can be refreshed for the rigors of the day ahead. Even dreaming is scientifically noted to be an important brain function that allows the brain to reset itself. We were designed for sleep cycles. An average sleep cycle is around 90 minutes and most adults need to go through approximately five of those per night. So get enough sleep and I guarantee you'll feel better in the morning.
Food - Well, food is a very large, diverse subject, so I'm really going to approach this from the "Amount of Food" standpoint. How much food is enough? How much food is too much? In an age of humanity where food is usually within an arm's reach it's fairly easy to overeat. How many times, in the last week, have you said, "Wow, I'm stuffed and I couldn't eat another bite." When you said that, you had just eaten too much food. You ate a portion that was actually unhealthy for you. And the scale goes up!
Let's take an easy look at how the human digestive messaging system works. Normally, you start out hungry. Then you start to eat food. It takes roughly 10 minutes for your brain to receive the fullness messages from your stomach. If you eat a meal more slowly, the messages arrive in your brain when the real time to stop eating arrives. If you eat very quickly, you can eat way more than your system can really handle and the message arrives to your brain saying that you've eaten too much (You're stuffed). Fourth simple rule of health: Don't eat when you're not hungry. Note: this rule does not apply if you have an eating disorder. An eating disorder is a psychological condition where your fullness signal from your brain is ignored by your will to eat (or not eat).
There are two situations I can think of that would exclude you from the "eat only when you're hungry rule". That's when you have a psychological condition that overrides your hunger. Those would be:
1) Over eating - Your emotions control your food intake or don't pay attention to how full you are. You can't judge a healthy weight for your body and/or; You can't judge a healthy amount of food intake.
2) Starving - You can't judge a healthy weight for your body and/or; You can't judge a healthy amount of food intake and/or; you purge your system of food in an attempt to control your body weight.
One way you can tell if you may have one of these two issues would be to take your current weight and check it against a suggested medical weight range for your height, age, and gender. If you fall outside the weight range, you may be over eating or starving yourself. Either of these situations are dangerous to your health and need to be addressed by a health professional... IMMEDIATELY!!!
Exercize - Exercise is the last item on my list of essential elements to live healthy. BUT, IT IS STILL AN ELEMENT THAT IS ESSENTIAL FOR HEALTHY LIVING!!!! Without exercise, your joints, bones, and muscles start having regular problems in your 40s. If you still don't exercise, the damage will become irreversible into your 50s. This may be OK if you don't care about feeling like shit every time you get up from a chair. Exercise is that one element that ties all of the other elements of health together. It has also recently been shown to combat depression which is so important in all of the things I have been saying here. Fifth simple rule of health: Get some exercise, take a walk, run on the beach, do some pushups, just move your joints... Do some activity that gets your heart rate up a bit for around 30 minutes... at least, every other day.
Keep these things strong in your life and you should have a healthy life.