Scanning isn't the hard part when trying to make pencil scans look good. It's going to be the photoshop in most cases. Make sure you're scanning settings are on the highest dpi like 600 dpi. After that go on photoshop and use the adjustments and "levels" and move the arrow on the left on the interface that pops up. Auto contrast and auto tone to see if it looks any better. If not Ctrl alt Z (undo).
If you want to photograph you're pencil drawings try to use sunlight because normal light bulbs might have too much yellow in them and make your pictures look awkward. Never use flash it's horrible.