Clean your scanner glass and photograph with a soft cloth before placing each item.
Do not scan at a lower resolution and then enlarge it later! This actually lowers the resolution and can turn it into garbage. For example, a 2.5″ x 2.5″ image scanned at 300 PPI that is later doubled 5″ x 5″ becomes 150 PPI. Photoshop cannot invent pixels that were never present in the image to begin with. So you need to scan something that will be used at twice the size of the original picture at twice the resolution (600 PPI at 100%). I’d rather you scan too high.
Black & white and grayscale are NOT the same type of file! A 1-bit black & white scan contains only black or white pixels; there are no shades of gray at all.
The bigger and cleaner the original artwork (such as a logo), the better it will scan.
Convert your color scans to CMYK color mode to get a sense of how they will look when printed – some colors, such as blues, greens and oranges, can change dramatically.
Grayscale images should be saved in grayscale (8-bit) mode. Avoid RGB or CMYK – it tends to add a magenta tinge to them.
Save your scans as TIF. If you use EPS files, save the image with an 8-bit TIF preview.
Do not open and resave a JPG mutliple times – every time you do, it degrades the image. If you plan to tweak the image and resave, save as a TIF first and use that file to edit.
MOIRE Crisscross patterns or checkerboarding in a scanned image. This occurs when an image was previously printed using the offset process (i.e. a picture from a book or magazine or newspaper) which converts the image into little dots. This may not show up on inkjet or even laser printouts at home. Moire problems do not occur with scans of actual photographs. One common occurrence is scanning an old CD insert you want to reprint. I do have a fix for this, but be aware that the fix includes slight blurring, so you should always scan from real photographs if possible.