Two options that have always really helped me.
A) stay awake until you're about to pass out, then go to bed then. You'll go to bed as soon as your head hits the pillow
B) Redraw your dreams. This may sound like hippie garbage, but you are in control in your head. You can change what is going on. There is a man with a knife, well replace him with cake, can be stupid, but you get my point. Understand what is going on, and then let it go.
Deep breathing and ambient music also help, if you focus strongly on one or the other, your mind won't wander to nightmares.