How can i help prevent cervical spinal stenosis?

You can't: The size of the canal is inherited. The secondary degenerative leading to further narrowing is also linked to genetics. You may be able to reduce this by healthy living (regular exercise, no tobacco use, etc), but overall you have little influence on this condition.

Don't smoke: If you have the congenital type, then that is just bad luck. Over time, our spines age & degenerative changes can lead to a narrowing of the spinal canal to a degree that compromises the nerve tissue & this is then called spinal stenosis. By not smoking, you avoid accelerating these changes & by avoiding neck trauma you can help minimize these degenerative changes that may lead to stenosis.