The first tip that can help you out the most is to make sure that
your dog knows that you are the boss. I know this can be hard, but the
way that I found to do this is by an exercise where you sit in a chair
with your Mastiff's leash in your hand and they are attached to it.
Then you will stay in the chair playing a video game, reading a
book, paying bills, or something like that while they lay down next to
you. Then you will sit like this for a half hour or so and only start
the timing count after they have laid down and decided to sit there. I
know that after five minutes my Mastiff would lay down and go to sleep
for the time so it was fairly easy.