I have an auto-immune condition that stress makes worse. I try to:

sleep well, exercise moderately, enjoy nature and walk a lot, eat food (not too much, mostly plants), drink decaf and just plain water, re-frame if the situation is stressful, try to increase resilience by positive stress (ie. cold water swimming, exercising, fasting).

I am still learning and I fail all the time. But then again, I don't stress about it :D