There are MANY different techniques you can try. The key is persistance, devotion, and well, remembering to do so.
dreamviews.com/f25/dream-stabilization-clarity-tutorial-95034/

If it feels like the your dream is ending, or becoming blury/dark, and regardless of how much you try to keep it, it still remains really persistant at doing so, -or- even in the dream, it is leading to some method of ending (such as going to sleep, completing a task, winning a match, etc etc) it can mean that you are coming to the end of you REM cycle, and your mind is getting ready to end the dream.