If I had to narrow it down to a couple, I would say patience and a good sense of humor. If you can remember that this too shall pass, and someday it will make a great story--it can help a parent get through frustrating time.
I have a story. I had one child at the time, she was about 18 months old. I wanted to take a shower. I set her in the bathroom with toys and rushed into the shower. Right in the middle of the shower, she starts to cry and gets hysterical. I am talking to her, trying to calm her and trying my best to get done fast. I get shampoo in my eyes, she vomits, slips in the vomit, falls and hits her head. It was one of those nightmare parenting moments.
This was 29 years ago and I remember saying out loud, "some day this will be funny, some day this will be funny." And we do laugh about it now-and my daughter survived!
If a parent can remember that most (albeit not all) of the trials and tribulations they experience with a child will pass - it can help tremendously.