I guess it depends on the kind of stressed. If the stress is from someone talking to me (read: lecturing at me), I'll get a tight feeling in my throat and find speaking very difficult, and I've been known to cry if it's bad enough.
If it's just the stress of life, I'll mostly just pace and rock and sometimes feel like hitting my head on things, which I've done when I was really bothered. Other times I'll clench my fists really tight and rub my head and face, or just stretch my arms out. A few times (the ones when I banged my head on things) I also pulled my hair.
God, I'm a mess. Anymore, I mostly just sit in my chair and rock and listen to some calming music, like King Crimson.
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"If knowledge can create problems, it is not through ignorance that we can solve them." - Isaac Asimov