I'm with the "they'll eat when they're hungry" thing, and also with the "don't offer anything else".
That being said, DS1 is thin, and I like him eating a varied diet. There are two tricks we find work in our house.
The first is bribery - he can have yogurt (sweetened and plain mixed together), if he eats X amount of his main course. I'm not totally happy with this, as it implies the main course is yucky, and dessert is good. But it does make him eat - and sometimes he doesn't even have room for the dessert, or he doesn't end up liking it.
And the second method is shouting. Not shouting at him to eat, shouting at him not to eat. "Don't you dare eat that! If you eat that, you'll be able to run very fast and I won't be able to catch you!" And then when he eats, I summon up ludicrous amounts of cartoon rage. I shake my fists, I howl, I shout. (Sometimes I stop to giggle. He knows, all the way through, that I'm joking.) This method has got him eating lots of things he wouldn't eat before, like ratatouille, or cabbage.