i know this might seem a little strange, but i have been having a lot of success with this lately. its really simple. i wrote the words "dont fail" on my hand, and everytime i want to give in and just eat or rest i recite those simple words in my head. it really gives me a boost anytime i need it.
i did a lot of walking today. i did 6 miles. around mile number 3 i just wanted to go back home, but i recited those words and it pushed me to where i wanted to be.
i wish all of you luck with your fasting, and i encourage you to find those simple words that will do the trick for you.