Here's an idea I just got from the cookbook "Deceptively Delicious"... If your kids are hungry before dinner, offer them some raw vegetables to munch on. If they're really hungry, they'll eat them. If they stuff themselves with vegetables before dinner and "ruin" their appetites, at least they got a good healthy dinner of veggies.
I just tried it with Eliana - offered her carrots instead of the traditional late afternoon popsicle snack. You know what? She took the two baby carrots and has been happily munching on them and not bugging me for a popsicle. It's a good idea for adults to remember too when you feel like snacking during the dinner preparation process!