Because celery isn't overpowering, it allows the spinach and cucumber juices to stand out. Spinach is a good source of calcium, iron, and potassium.
Cut large produce into chunks, and remove big seeds or pits. Don't worry about those in fruits like apples and pears -- the juicer will filter them out. To combine several fruits and vegetables, alternate between soft pieces and hard ones. Finish with the latter to push through anything that's stuck.