Always Sleep With a Pillow
Sleeping without a pillow deprives your neck of the support it needs during the night. Without one, your neck will have to work extra hard to support the weight of your head, which puts severe strain on your neck muscles. There is actually no possible position in which you can sleep in without a pillow that won't strain your neck, and the only way to avoid this unnecessary neck strain is to use at least one pillow.
Limit the Number of Pillows You Sleep With
If you sleep with more than two pillows, you're probably setting yourself up for a stiff neck in the morning. Stacking up your pillows too high and then trying to sleep on top of all of them will cause a stressful imbalance between your neck and spine that could throw your neck vertebrae out of alignment. As any chiropractor will tell you, stiff necks are the result of your vertebrae alignment going awry. You'll almost certainly have that annoying pinched feeling in your neck muscles in the morning, and you may even have trouble turning your head.
Get a Pillow That's Supportive
Not all pillows are created equal. Some give your head and neck better support than others. Pillows that are too thin are almost as bad as sleeping with no pillow at all, while the stuffing in old pillows may be unevenly distributed from too many washings, resulting in empty spots in the middle that will stress out your neck. Hard pillows don't have enough "give" in them to properly support your neck, and can result in headaches. The best pillows for preventing a stiff neck while sleeping are of medium firmness, with new, plump stuffing that still has some bounce to it. If you can find a contoured neck pillow that's designed specifically to fit the curves of your neck and head, you'll do even better.