How to keep somebody from snoring?

Depends: Most snoring is worse when on the back. Also if you've been drinking. So avoiding alcohol and not sleeping on the back seem to help. Also making sure they can breath through their nose also helps. They may have sleep apnea as well, so seeing a doctor for evaluation of this would be a good idea.

Snoring: If you are overweight, losing weight not only reduces pressure on the airways, the tongue shrinks in size, further opening the airway. Don't sleep on your back ( sew a pocket in a t-shirt and put a tennis ball in it ). See an ENT to make sure that there isn't a problem with deviated septum, tonsils or adenoids. Use breathe rite strips. See a dentist to have a custom sleep device made.