Just watch them. The cower when they realize that you are not going to fight. I know I sound like I'm talking about a school bully or something, but IMO they are similar.
Keep an observer perspective during your fast. Your mind can be very conniving when it wants you to do something. Your mind will try to make you believe that you are it. But we are not the mind. Not to get preachy or too Buddhistesque here.
Just remember that you are not any of the thoughts you have. The self defeating ones, the cravings...observe them. I'm sure you got it the first time I said it, but I'll say it one more time, they cower quickly when you simply watch them.
Once you are firmly grounded in this observer state, fasting almost becomes entertaining, because the ways your mind will try to get you to quit. I find it a bit humorous.