Hi Virginia. Little suggestion, buy a pop up reflector panel, or simply use a large piece of cardboard with silver foil glued to one side to bounce light into where you want it. Cheap, simple and effective for adding that extra little WOW to images, and softer then flash.
To me this is the best of the series,though shot in a harsh light love the highlights on her hair,agree with Captive regarding use of foil board or can use white thermocol as a reflector or since you are using D90 shoot in manual mode & open aperture a little more to lighten her face.
It's easier with an assistant, but if you keep it light you can do it alone. Remember the camera can go on a tripod if necessary, with a remote release. You can buy one handed ones, but they are expensive, hence the cardboard and aluminium foil version.
Not sure about the D90, but the D70 and the D80 could use a programmable TV remote as a remote shutter release, tuned to one of the Sanyo settings! The power on/off button becomes your trigger.
They have the remotes on ebay for almost nothing (from Hong Kong). Like Miles said - the reflector would make a nice change.
I know for my camera I can buy remote releases that are anywhere from 2-25' long, more than enough to be able to hold either a reflector and/or additional flash. I have used the wireless remotes but have found the inexpensive ones unreliable.
This is probably my favorite pose - this or the newest upload.
Plus you have to either remove the battery or put it somewhere where the buttons won't be pressed down (like inside your camera bag - I couldn't slip a piece of paper extra into mine without it being squeezed between other things).