The "blocky" look may be an artifact of resizing a "halftoned" scan, or of applying too much smart stuff (auto-this and -that) to a halftoned image. As CJM notes, scan at 72 or 150 DPI, into an overall image size of 300x300 or 600x600 pixels; turn off "automatic" features in the scan software (auto-descreening, auto-sharpening, auto-screening) -- just make a straight "text and graphics" scan. If your initial scan doesn't produce a square image, just use an image editor program (perhaps the scanning software will do the job) to crop -- not resize -- the image to square dimensions.
If you're scanning to jpeg, keep the quality settings very high right up until you're done cropping, color correcting, unsharp masking, etc. -- make reducing file size by lowering the qaulity setting the last thing you do. (And if you're doin' it up like downtown, scanning to TIFF, keep the image as a TIFF right up until you're done editing; convert to jpeg as your last step.)
