Trouble with little girls who won't eat

I just don't know what to do any more... it's so frustrating.

My 5 year old daughter Emmalee hasn't been eating much lately.  I've tried it all:

• Let her pick out what we have for dinner
• Let her help with the grocery shopping
• Let her help prepare the meal
• Let her pack her own lunch for school

Still, 3 out of 5 days at school she brings back her lunch box almost completely full.  At dinner she takes a bite or 2, and we have to practically force her to eat more.  

When she was younger, she would literally stuff her face with food and refuse to swallow so she wouldn't have to take another bite.  

I know she's hungry - she says she wants snacks after school, up until dinner and after dinner.  But even if she picked the meal, she still won't eat it.  It's affecting her behavior and her sleep schedule.  She does eat a lot at breakfast, and I do sneak some extra nutrients in her with strawberry protein shake mix in her milk after she's refused dinner.  

She has favorite foods (spaghetti, tacos, sweet&sour chicken); I try to make them often for her.  She likes vegetables; she'll eat a few when I put them out with dinner.   

SO FRUSTRATING!!!!!

So parents - what do you do to get your kids to eat?