Believe it or not, the impact of dirty, dusty, insect-laden globes, diffusers, reflectors, or shades on your fixtures' light production and efficiency is enormous. So, if you're aiming for efficiency, get up there and clean them off!
For reading, place lamp alongside your shoulder, with the bottom of the shade even with your head. Extra tall lamps should be moved back 2 feet if possible. Use several lamps in a room to avoid shadows and dark areas. (One standard lamp will illuminate 40-50 square feet.) Desk lamps should focus light on the work surface and be adjustable. The light should be about 15 inches from your desktop. You generally want to position the lamp 36 inches or less from the item to be lit. Be sure your lamp is in proportion to your room and furniture. (e.g. A small end table should hold a small lamp.) Chandeliers should hang approximately 30 inches above your tabletop and be 6 inches narrower than your table on each side.