Well, when I'm asleep and something scary is happening, I usually try to control it and change my dream. When Ты see something off and completely impossible in a nightmare, just remember that Ты can control it and it is just a simple fear in your mind. NEVER let it bother you. (: If Ты let it bother you, it will come back. Sorry if this doesn't help. Just throwing it out there.
Read a good book that wouldn't produce nightmares is good. Also, if you're a Christian (or any other religion) read your Bible and pray. Eat ice cream или any kind of sweet, really. This usually helps me, I don't know if it'll help you. Think of all the good things that happened that day.