some good precautions to take...
change your pillowcase once a week! oils build up on that and your cheeks rest against it 7-8 hours a day.
also, pimples on the forehead can be caused by the way you shower....keep your head up so that the oils from your scalp aren't running down your forehead.
other than that, never go to sleep without washing your face first, cleanse in the morning and use a good moisturizing SPF.