You have a VERY good camera and the problem is much more likely to be user error than equipment shortcomings. I don't mean to be harsh, but want to be honest. To get the best out of your camera there is a lot to learn. It may seem daunting, but take it slowly and methodically. It will eventually make sense.
Bryan Petersons book is a good start to learn about exposure. For the camera itself, I found the
Complete Guide for the D300 by Thom Hogan an excellent source to put into practice what you've learned from the exposure book. The Guide explains the settings and how to get the most out of them as well as a slew of technical information that will benefit in its on time....... when you're ready for it.
Don't be afraid, shoot RAW and use a program like
Lightroom 2 to enhance the file in post.
Try stopping down to F8, get a good shutter speed, set proper WB, and set picture controls to vivid, and increase sharpening.
Stopping the lens down to f/8 is likely only getting you to the sharpest f/stop but may have nothing to do with the DoF you want to achieve. Setting in-camera edit controls may be useful if you have to shoot and post without any additional processing. I would much rather set controls to neutral and judge for myself what the final product will be. Understanding the triangle of exposure is a preferred method IMO.
It sounds like you are shooting models (in a studio?) so prime lenses may be worth investigating. Something like the 35mm f/2, 50mm f/1.4 or the 85mm f/1.8 (better is the f/1.4, but geesh, the cost may be prohibitive). I think Sigma makes some good copies of these focal lengths, but I have no experience with them. I can say the Nikkor versions are very good.
Good luck and please feel okay to post your photos.