I'm going to give a controversial answer here; get lucky genetically.
Some people can smoke all their lives and never get sick from it, some people get cancer despite living a healthy life. Ozzy Osbourne famously had his genome tested, and he found out he's basically indestructible when it comes to drugs - he can take an amount of drugs that would kill most people. Your general health has already been pre-determined genetically. It may be depressing, but your future health was determined at the moment of your conception, your mix of genetic material more or less determines how well you will live. Of course genetics isnt an absolute measure, environmental factors play a role (smoking, drinking, risk taking, diet etc) and the degree to which genetics is responsible for your health is debated.
I'd like to think that in the near future, comprehensive genetic testing will be available for everyone, which will allow you know your genetic weaknesses, eg if you're genetically predisposed to liver disease, you know not to drink alcohol, but obviously there's ethical and moral issues with that, and it's far too expensive to do on everyone at the moment.
So to answer your question, I don't consciously keep my health, my body does. Having said that, it's wise not to drink too much, it's wise not to smoke, it's wise not to eat junk food etc. I'm not for a moment suggesting anyone lives a very unhealthy lifestyle, but life is unfair, and some people will just be more healthy than others.
