Resizing windowed mode

What is the proper way to make a Direct3D8 window resizable? So far It creates a window 800x600 and sets the back buffer width and height to zero in the present paramiters In the window proc it detects when it resizes and gets a RECT using GetClientArea. This updates the aspect ratio It works, but is is blurry! How do I prevent it from resizing the back buffer when it copies the image???