[Utah-astronomy] Flat Field Frames

For those of you that do CCD imaging I was wondering how you take the flat field frames. I've heard that that a twilight sky makes a nice flat frame but the camera has to be focused first. Without stars to focus, this is quite difficult. I was going to try putting some printer paper over the dewshield but I filled up my memory card before I had a chance to try it. SBIG recommends a white T-shirt. Has anyone tried this before? I'm looking for ideas how to incorporate flat field frames into my images to remove dust and other artifacts that accumulate on the lens. Clear Skies, Debbie