Our 4 yr old is a pretty good eater but some times she doesn't always want to eat (when we know she is hungry).  So when we need to get her to eat something, even if it is just for a few bites we use reverse psychology on her.  We start eating our food and tell her, "Oh you can't finish that can you?  I bet you can't." and she'll say, "Yes I can. Watch me. " and all of a sudden she will start eating. They will respond to this vs. nagging them to eat.  Or sometimes we will change it around and say, "Well if you're not going to eat it, then we'll give it to your sister." and of course she starts eating.  Another thing that works is we will tell her how many bites to take and she will be focusing more on the counting so she is distracted.  We don't yell or nag; we just say it in a very non-chalant mannner as if it is not a big deal and it always works. Also for both of our kids, we don't put a lot of food on their plates because we find it can be too overwhelming for them to feel like they have to finish all of it.  So we give them a little bit at a time and if they want more they can have more and it also helps make them feel like they have some control which they like.