Hi Amanda, it's been a long time. "Keep calm" is a correct and common phrase. In fact, I immediately thought about the famous "Keep calm and carry on" posters produced by the British during WW2. http://en.wikipedia.org/wiki/Keep_Calm_and_Carry_On Keep calm, stay calm and be calm are all very close in meaning but if someone takes the time to think about their usages, there are slight differences. As always, isn't it? haha. Keep calm and stay calm are closer in meaning to me. When you say that to someone, you're asking them to remain calm and not panic. When you say "Be calm" to someone, it's the same thing but it is more authoritative. It sounds like an order like "Stand up!" or "Sit down."

I've already heard 'keep calm' in a film, it is right and common.

I'm no native but as far as I know keep calm is correct and very common. I guess it means : GET calm and STAY calm as the verb keep has also this meaning of intiating and remaining. I've googled keep quiet vs stay quiet recently and it was quite illuminating. : ) As for the second question I would say something like Think hard or Think carefully before making this decision.There must be many other ways to express that though.