Keep a notebook/journal/folder–whatever suits your style–in which you record your especially vivid or disturbing dreams. Record the dream as soon after the event as you reasonably can, and include as many details as you remember, however bizarre, disjointed, or impossible they may be. You can make use of these dream records in at least two ways.
The most obvious way to use these dream records is when you need your character to have a dream. You can either lift it in toto or use it as a starting point. Much easier than creating a dream out of whole cloth.
Because dreams often contain odd just0positions, they also are useful when you are writing something that calls for a supernatural, mysterious, or merely unexpected series of events.
Once you are in the habit of collecting your dreams–and maybe the dreams told to you by family or friends–you will find yourself using them in surprising ways.