Although you may not be able to completely prevent acne, there are steps you can take to improve your skin.
Gently wash your face twice a day using a mild, pH-balanced cleanser (Cetaphil or Dove bar) and warm water. Use your hands rather than a towel or other object that may damage the skin. Removing dirt and excess oil decreases the chance of skin peeling, drying, and cracking. Avoid scrubbing the skin harshly and any oil-based skin products that can clog the pores. Use hair products, moisturizers, and cosmetics that are "non-comedogenic," as they are designed to keep your follicles and glands from clogging.
There's no medical evidence that acne is caused by what you eat or your personal hygiene, although stress can sometimes worsen acne.