Personal opinion. Take it or leave it.
  1. Do it in person whenever possible.
    Even if it inconveniences you.
  2. Be honest.
    Tactfully so. In the long run, the honesty matters and helps with the moving on.
  3. Don't do it over text.
  4. Don't do it after taking the person to see The Break Up (or modern day equivalent).
    Yes, this happened.
  5. Don't beat around the bush or make up a story
    You might think you're softening the blow; it just makes it worse.
  6. Don't rub the persons nose it in immediately.
    If you move on the same day, don't post it on Facebook. Again, remain tactful.