Studio lighting for white background

I am a photographer (lifelong rookie with 4 years pro) and am starting studio portraits. I have a black and a white backdrops. I have two umbrella lights, and a nikon d70 camera and an 18-70 lens. My question is this. What should I be shooting at to get a a clean, crisp sharp image, without having to go into photoshop and completely gut it. I am working in a 13'x13' room. The size of the room shouldn't have to do with the crappy pics I am getting, so, please tell me what I am doing wrong. I have changed my f-stop, aperture, the strength of my lights. Is there a setting in my camera I should be playing with? Like white balance or something? Any advice would be great. Below is an example of what I am trying to acheive (courtesy of rebecca knowles photography). Is this more photoshopped than studio, that you can tell? Or is there something I am missing in the studio. I just want to create a portrait that I can get such a crisp, cleancut, sharp, bright look out of. Help anyone?

strobes or hot lights? It would help if you posted your own photos so we can tell what you're getting and make better suggestions. I assume you're shooting in manual.

Thanks for answering. However, I found my answers on a different forum on another site. I really appreciate you reading my question though.

jezziemae - As you probably already found on the other site they usually use two strobes just behind the subject aimed onto the white background to over-expose it. That creates the clear, white look you see on fashion shots. Then another frontal strobe illuminates the model. Check out this effect in this uploaded shot.
-GB