How do you make your boss understand your migraine condition?

Jobs are hard enough as it is. Add in having to show up late, leaving early, or not coming in at all due to a migraine and many are left unable to advance in their careers, or worse yet, without a job. If you have successfully spoken to your boss to make them understand your condition, please share how you did it and what you said. If we can effective communicate what is going on with our employers, hopefully, it will increase understanding. So, what advice do you have to share?