Exercise
Regular exercise greatly increases your own well-being. It improves physical health and makes you feel better mentally as well.
I exercise in the morning right after I get up, because it makes me feel active and ready for the day. That's why I prefer the 7-minute workout. Then in the evening, I either go for a run or do some strength training.
Stretching
You could include stretching in the sports category, but I think it's too important a topic for that. Daily stretching ensures that mobility and freedom from pain will last into old age.
When you're young, this habit may not have as many benefits, but it will pay dividends as you get older. You should start investing in your health now - especially if you sit a lot at work.
Sleep
On average, a person needs eight hours of sleep per night. However, this number varies between 7-9 hours for most people.
Figure out how much sleep you need per night, and then make sure you get that number on a regular basis. The best-case scenario is that you sleep long enough every night.
Exceptions to this rule should be well justified, because the after-effects of getting too little sleep can still be measured weeks and months later.
Healthy Nutrition
Exercise, sleep and nutrition form the trinity of physical health. The topic of “eating well” is a very broad field with many opinions, experts, and conflicting advice.
For a first overview I recommend the short book “Food Rules” by Michael Pollan.
Relaxation
So far, all tips have been mainly related to physical health. Of course, physical survival is a prerequisite for growing old. But what good is a healthy body if the mind is not comfortable in it?
That's why you should pay attention to your mental well-being in addition to your physical health. This can be achieved through mindfulness, meditation and relaxation.
Growing old doesn't mean having more time to work, but longer to marvel at the nature around you, other people and the whole universe. Keep your curiosity, adventurousness and inner peace. Because besides physical health, these characteristics are one of the few things that money can't buy and sheer willpower can't achieve.