"Be angry and do not sin ..." Ephesians 4:26 (ESV)
"We should admit that we are angry, but we should never use our anger as an excuse to sin."
Everybody gets angry at some time. The only difference is how we deal with it: some people blow up, other people clam up. Some people express it, other people repress it. The most important thing you can do is admit you're angry.
God doesn't forbid anger. In fact, in today's verse God says, "Go ahead and get mad." That's a pretty easy command to follow. Sometimes anger is a valid response to life. In the Old Testament, God got mad. In the New Testament, Jesus got mad. He got mad at the hypocrites. Anger is a God-given emotion. It's emotional energy. If you're never upset by anything, you'd better check your pulse. You're either dead or you're not in touch with reality.
Some Christians get false guilt thinking, "I should never feel angry." That's not only unhealthy, it's unbiblical. God says, "Be angry, but when you are, don't sin." We should admit that we are angry, but we should never use our anger as an excuse to sin.
Isn't it amazing that we hate to admit it when we're angry. "I am not yelling! I am not angry! I'm not mad!" -- and the veins are popping out! We deny that we are angry.
But the fact is, internalizing your anger does not make you a better Christian than the person who blows up. Either way, it's wrong and it's an ineffective way to get others to understand your point-of-view.
I believe if most people obeyed this verse, we'd have a lot less depression in our society. The number one cause of depression is repressed anger. If you don't talk it out, you're going to take it out -- on yourself or somebody else.