Is it a last minute deadline? worries about money? an argument that you had... what is your reaction to stress?

This, I believe, is when the yetzer hara does its best work. How does the yetzer hara manifest itself and what can we do to combat it?

Personally, I know that when I get angry I have the most difficulty staying calm - a most important thing especially as a mother. I have found that when dealing with my children, that I stay calmer if I sing what I want to tell them, instead of speaking - or raising my voice - which tends to be instinctive and yelling.

I am sure that I am not the only one with this type of problem. I know that it is difficult for others to take mussar from someone else, but as a parent I must also give mussar to my children. I have one child who absolutely blows up when faced with a difficult situation. I know an adult who loses his/her temper and yells and swears every time there is stress.

What can be done to build oneself to be a better, calmer person - a stronger one in the face of the yetzer hara?

The Yetzer Ha'Rah is an easy scapegoat, and a very vague term. By that I mean it is accused of all sorts of things we ourselves do not take responsability for and thus, we all entertain a romance with the Yetzer Ha'Rah!
Controling one's moods is certainly difficult but a loss of control stems from a lack of discipline, stress, fatigue, disappointment, frustrations, as well as all sorts of drives and desires. If they are not taken care of, they will burst later, in unwanted ways...
The Rambam does not mention "Yetzer Ha'Rah" in his Code of Law, but in Halachot Deot he gives instructions how to dominate and direct one's personal tendencies through constant self-discipline, in order to acquire good habits.
The Sages say: "The Yetzer Ha'Rah has a strong desire to do what is forbidden" (Tractate Yoma) so the anti-dote is to stop this desire before it becomes overwhelming.
It seems to me that if you build your son's self confidence on a daily basis, he will not have the need to blow up when frustrated, and if he has a bubbling temperament, some natural medicine to calm him could help stabilize his moods.
Everything I have said to you, I also say to myself!

Thought I'd share this neat experience: When the girls were very young (they are 10 and 11 now) I read a midrash that said if you must strike a child, do so with a shoelace. I felt this was what G-d wanted for my children and that corporal discipline was an unneessary tool in their case. So I committed to this. One day, about a month ago, my little girl, Yemina was in a "funk". She gets really moody sometimes and was really barking at her older sister, Zipporah, who is far more easy going. I had had about all I could take with her yelling at her sister and I went to my bedroom. A few minutes later, Z came in and was crying about something Yemina had said to her which was really ugly and demeaning and not worth reporting. I told her to send in her sister.
A few minutes went by and I knew my blood was boiling. I love a peaceful, quiet home, and Yemina was systematically dismanteling it. Then I prayed, and I decided not to do all the typical, "Straighten up or you will find yourself grounded, or eating by yourself, and so on. I abandoned all of that and asked G-d to give me wisdom. A minute later she appeared, sulking, arms crossed, her beautiful features twisted into a sour scowl. I didn't say anything because wisdom hadn't come yet. I didn't know what to say without my arsenal of tricks. So it became quiet and frankly awkward and uncomfortable. Then, I said (very gently), "Yemina, I think you might be out of balance. Your yetzer ha-ra has been running unchecked lately." I looked at her, she began to rock but clearly had nothing to say to me. Then I said, "you can feel it can't you?" She bowed her head lower and started to sniffle. I went to her and held her with all the love in a mother's heart. I told her I loved her so much, and that it made me hurt when I saw her hurting and that we could pray together and start to feed the yetzer ha-tov in her by purposely doing good things so that it would grow stronger than her yetzer ha-ra, and that pretty soon, it would be so easy to control her bad impulses because they had been kept under check with discipline. She held me so tightly I thought she'd pinch me in two! And she wept a truly penetant and heartfelt cry for help. Ever since that day, things have never become so out of hand. I remind her of the inward check-up and she knows what is going on. I really think she didn't know why she felt the way she did, and her anger and confusion was part of her lashing out. I am really glad that MY Yetzer Ha Ra didn't stay stirred up when dealing with her, because I would have missed a a real pinnacle turning point and a very tender moment. For what it's worth-- Shalom! Yocheved
P.S. To Sheva-
Yetzer Ha-Ra, in my experience, has always been just that, an inclination to lose control and to do what is forbidden. I teach my children that the two yetzers are born in you and that it IS your responsibilty to control them. When one impulse is noticeable stronger than the other and far more difficult to control it is because you are indulging it. The way to get one's balance back is time with G-d, disciplining one's body and mind to DO GOOD which reverses the affects of an over-indulged Ra. Blame has never been on the radar in our discussions. Discipline has, on all levels. And i agree, if someone uses it as an excuse, then they clearly don't understand the meaning of the words. Sincerely, Yocheved