Pillows have a greater impact on quality of sleep than most people realise. Depending on the way you sleep and whether or not you have shoulder or neck problems, a flat pillow may suit you better than a fluffy one. If you sleep on your side or back, a fluffy pillow provides good neck support. However, if you sleep on your stomach, a flat pillow is more beneficial for you. Pillows used as seat cushions should also be flatter than their decorative counterparts.
Use the pillow for a while -- it will naturally start to lose its loft after a few weeks.
Lay something heavy on top of the pillow. A large book, a heavy box or a couple of bricks wrapped in a towel will flatten a pillow after a few days.
Cut the pillow's seam on one side with a seam ripper and pull out as much stuffing as necessary in order to make the pillow as flat as you want. Sew the seam closed.