“But I’m not hungry…”
“You’ve hardly eaten any of that dinner”.
“But I don’t like peas”.
“You did yesterday. Remember, there is no dessert unless you finish”.
“But I’m not hungry FOR dinner.”
“But you’re hungry for dessert?”
“I will be later, when we have dessert.”
“All right – how old are you?”
“7.”
“Right – eat 7 more mouthfuls and then you can leave the table, and have dessert.”
“Ok.”
Get Up Before You Fall 
I must try the whole age / mouthful technique……..all i ever get is “Im too tired to eat” off my youngest, this is of course while shes running around like a lunatic!
It worked on all of us! I will not admit at what age it stopped working… Although we still joke about it now!
That is one tough negotiator.