I always think if you can get to day 4 you have beat it. Day 5 onwards are easy compared to the first few days. Just stick to it, however bad it feels.
I find I have a splitting headache and total preoccupation with food for day 1 to 4 (seriously, I drool over adverts, which seem to be related to food all the time), and then it just goes calm and I couldn't give a toss about not eating.
Good luck