"The pursuit, even of the best things, ought to be calm and tranquil."
- Marcus Tullius Cicero

"Never be in a hurry; do everything quietly and in a calm spirit. Do not lose your inner peace for anything whatsoever, even if your whole world seems upset."
- Saint Francis de Sales

Keep your cool / Keep your head (Idiom) - Remain calm in a difficult situation
http://idioms.thefreedictionary.com/keep+head

TIP 1: Count to ten when you are angry. This will calm you down and give you time to think.