Those sketches really are amazing!
(Do you mind my asking what medium you use to get things to show up on the Internet? My original sketches are much more detailed than anything I can get to show up when my drawings are scanned, so they don't look nearly as good here).
I have the same problem with my scanner--especially because the drawing are larger than the scanner (that one is 20" x 26"). I have to scan them in section by section then paste them together on the computer.
By the time everything is where it should be, the finished picture is sometimes a little blurry.
I get around this by saving my progress frequently, closing the editing program, then re-opening it. I can't explain why, but doing that helps maintain the resolution of the picture.
I also try to keep the scanned sections as large as possible--around 150kb--when I'm working with them. The finished picture is usally 400kb. I shrink that for posting purposes, then use the editing program to sharpen the image until it looks as much like the original as possible.
Whew :)