First of all stress is very important as a mechanism so "having no stress at all" is not really possible, but being constantly stressed is the way to burnout. And it's not about working 12 hours a day .... it's about working 12 hours a day but not wanting to.
Depression and burnout for example are stress related sicknesses.
So what can you do to work against it ? I think there are loads of strategies but I just want to explain two simple techniques, how the brain works and why it's working.
.... take a long walk ;) -> and while you do it try to think about something else -> you change the environment -> you exercise -> it calms your brain and in the end that's what you need.
Important about this is try to think about something else and changing the environment. There are loads of special techniques, but that's a simple one. It works if you're angry as well as on every other heavy emotion.
It's important to get distracted from the emotion ;) and afterwards ... talk about it that's as important, but afterwards not while you're in it.
Another technique is "fake it so it becomes real" the body and the brain have a bidirectional communication so .... if you fake certain muscle movements it triggers the emotion. It's used in laughter yoga but a more fundamental method is described by Vera f. birkenbihl.
Laugther will reduce the stress, so it's the best way if it occurres natural by having a friend keeping your spirit up - one good point why it's very important to work with people you really like and trust - if you don't have this natural, you can still just fake smile for 1 minute straight ;).
it triggers the brain to release the chemicals :) this will help you in really stressful situations to get back to normal ;D ... but don't do it in public it tends to look weird.
Besides this I think Hipkiss nailed it with communication is key as well as Todd who mentioned putting things in perspective and actually stepping away from the problem :).
Stress will always surround you and you solve it the same way regardless what you do ... oh and don't confuse fun with happiness .... ;)