A few things I've found that work for me.
Walking away. Just realizing that whatever it is... it's not worth my losing my cool over.
I keep a package of European-style cookies that I love (LU) in the cupboard. I'll make a cup of mint tea...take 2-3 cookies... go to my room and escape for 15 minutes. Sometimes, I'll even turn on the TV for the kids to lengthen my time of peace. If I'm really frazzled, I'll even put on TV shows that we don't regularly watch. IMHO, the break and peace is worth the quesitionable material on occasion.
Taking a shower.... or shoving all of the kids into a bubble bath (well, not the baby).
Surprisingly, letting my kids know that Mom is having a tough day. I used to be a massage therapist...and probably because of that, our entire family massages each other. So if I say that, the kids will climb up on the back of the couch, and massage my back.
Family sandwich. I lie on the floor...I'm the bread...then my eldest gets on and decides what he is (peanut butter)....then my middle son (jelly)...then my daughter (bread on top).
Try to get them to be silly (or do this myself.) We were having an issue with DS1's anger. He would get really angry at everybody--and couldn't calm down. So if he said something really loud and mad... we'd then ask him to say it in a grumpy voice... then in a baby voice... then like Foghorn Leghorn.... then like xyz.
Onetime when I was pregnant with DS3, I remember just walking outside the door and closing it and breathing in the nice fresh air. That was wonderful.