These are the big picture items.
1. Maintain your sense of control over your own life.
This doesn't mean being a control freak - trying to control others. It means having a good sense of what you are able to do to stay healthy and create the life you want.
2. Maintain good networks.
The better your friends and networks the more likely you are to be happy and healthy.
Doing these first two will mean that you have a good chance of handling stress, which is the cause of much illness.
3. Get some exercise.
Exercise regulates appetite and helps with weight control.
Aerobic exercise (deep breathing for more than twenty minutes) helps keep you heart and lungs healthy. You are less likely to die of a heart attack.
It doesn't need to be very hard. There is no evidence that elite athletes live longer than people who are normally healthy. Aim for a half hour a day that makes you breath more deeply. This is technically 70% of maximal heart rate. If you estimate what it is for you to "do a seven" (where zero is lying down and ten is running flat out) this is usually a pretty good guide.
4. Eat healthily
Eat until you feel 70% full. Wait 20 minutes until you eat any more. It seems to take this long for us to know whether we are satisfied.
Emphasise fruit and vegetables in your diet. (This is for those who eat in the usual way.) There is some evidence that organically grown may be healthier.
Cook at home when you can. You are more likely to eat less processed food and so you will avoid the preservatives and added fats and sugars that are so much a part of packaged food.