Hi DeHe,
My LO also has nightmares, and I did as a child too. A few weeks ago he had one that his brother died

That was a really hard one.
I second the whole idea of trying to turn the nightmare around. That helped me.
I have a hypnotherapy CD that really helped my LO. It talkled about strategies for dealing with nightmares and things. DS listened to it as he went to sleep and it really helped him.
http://www.mindmotivations.com/shop/products/overcoming-fears-amp-nightmaresAlong the same line - Dream catchers.
Basically antything that he believes will help keep nightmares away will.
Playing very soft music, like a sleep CD, while he is sleeping sometimes helps - and gives him something to focus on while he is trying to get to sleep. (I still do that today to help me sleep.)
Lavender essential oil.
My LO and I also seem to get more nightmares when we get overheated? No idea why that is?!?! But I have really noticed it.
And not quite on the same thing, but I found that the effects of a nightmare can last for days, making me feel 'weird' and unsettled. Just so you know, if the same thing happens to your LO.
Hang in there :P