The other comments here have dealt with the physical issues causing stress: poor nutrition, lack of exercise, and sexual activity. These are all things which can make it hard to be mentally on top of your game. That said, there are mental exercises/strategies to deal with situations that can reduce stress.
So I think stress is mostly internally generated. What do I mean? If you think about most behaviors and why you do the tings you do, it's because you don't like the outcome.
Examples
speeding --> speeding tickeds
slacking at work --> losing your job
eating lots of crappy fatty food ---> obesity etc.
I often stress out when I have a bunch of things I want to do and not enough time to do them. So prioritizing is key. What happens if you don't do it? Do you lose you job? Do you get in a car accident? You gotta separate the urgency from the importance.
4 types of tasks/events:
1) urgent and important
2) non-urgent and important
3) urgent unimportant
4) non-urgent and unimportant
If all you get done are 1 type tasks, then that's pretty good. 3's and 4's are the ones that tend to add to the pile, but really have no impoact on productivity and lifestyle, they just add stress. Let them be, focus on 1's and 2's. Everybody gets wrapped up sometimes, which often makes even more stress. Be gentle with yourself-- if you have an unproductive day, let tomorrow be a different, independent day. You don't have to make up for yesterday, that just adds more stress.
Live now, live well.