Outdoor Flash Portrait Photography?

So I have a portrait shoot soon with a friend and I'd like to try out this technique I've heard so much about.

Supposedly, it involves over exposing or underexposing the background and using the flash to expose your subject. I've seen how some of the photos look really dramatic with low key and high key lighting. I've seen and admired the "Invisible White/Black Backdrop".

Here's my gear which I will list:
Nikon D90
Nikon D5000
18-200mm f5.6 G VR
50mm f1.8 G
SB-700 Flash (comes with diffuser, bounce card, and color gels. I have no other third party diffusers)

I'd like to really achieve good technique and great results. I want to use my D90 with the 18-200mm and the SB-700 flash to do the outdoor flash portraits and use the D5000 with the 50mm f1.8 G as a backup camera and get other great portraits using bokeh.

Thank you for your time and any input would be greatly appreciated.

Added (1). @pk: Not only am I trying to achieve the black background but I would also like to get a white background also. Thanks for the advice.

Added (2). @ pk: By "this technique I've heard so much about" I'm talking about outdoor black or white backgrounds, and outdoor flash portraits in general.

Not exactly sure what your question is but from what I read I guess your trying to take a portrait shot outside with the subject only being exposed and the background black? The best way to do that (from what i've read up on as i'm attempting this kind of shot also) is to set the camera to manual then set the shutter speed quite high such as 1600/1 or faster have your aperture number quite low and ISO of around 100. Once this has been done make sure that you only light up the subject with the flash using a soft light rather than hard light. You may want to test it on some other objects first before the shoot just to be on the safe side

Simple… Your camera have a light meeter either through the view finder or on the lcd screen… Shoot in M mode and set your aperture lower or higher to over or under expose the background and then use your flash to light your subject.

This is called overpowered flash, overpowering the ambient, or overpowering the sun depending on where you hail from apparently!

Not difficult to do… Just take a meter reading of the scene, then set your camera to underexpose by 2 Stops. Then set the flash to overexpose by 2 stops. This will result in a properly exposed subject but a dark background. I also like to back the flash off slightly depending on how light the subject's clothing is.

You can't use the same technique for a white background. You need to light the background separately and overexpose it by a stop or two, while correctly lighting your subject with other lighting. You must keep your model well away from the background for this form of lighting to prevent any back scatter or spill onto the subject.