1. Setup a list of current projects
If you find yourself starting a new project over and over without completing the one before you are not alone.  It's my favorite thing to do.  I can share with you 2 ways I've just learned to keep from doing this.  I have incomplete crochet projects scattered all around my house. Maybe it's just boredom or 'out with the old in with the new' syndrome. Whatever the case I have started to see a couple of ways around this habit.  First I set up a list of recent projects to document which projects that I am working on. This way I can log my progress and stay on track to get at least one of the projects completed.
2. Visualize the finished work
Secondly, I keep looking at the picture of the final project, thus creating a visual goal to keep in mind.  More on this later.  Hope this helps.