hun it can be really hard with fussy kids and everyone has given you some great advice.
DD1 used to eat anything you put in front of her and now i have big problems with getting her to eat her dinner, she will graze all day (given the chance) and would sometimes demand certain foods which really weren't great for her.
At the moment the things i do to try and get her eating a bit more of the good things:

1. Buy 1 pack of biscuits for the week, if they go they go and that's it - NO MORE
2. Plenty of fruit & veg - the girls eat fruit like it's going out of fashion, they will also help themselves to carrots in the fridge and even though DD1 wont eat them i ALWAYS serve her vegetables in or with her meals. DD2 will eat veggies, DD1 wont.

3. Salad - i make a salad, pick out the things i know the girls eat and put majority of that on their plate PLUS a tiny bit of the rest of a salad, DD1 is finally starting to eat lettuce as that's what BIG GIRLS eat and she's a 'big girl' now

4. Fail Safe Meals - i've recently found i generally have 2 fail safe meals - bolognaise and Taco's / Wrap's - in the bolognaise i hide zucchini, carrots, onion, capsicum and sometimes peas & corn and the girls don't even notice and Taco's i just find they like building their wrap or taco so they eat it.
Where possible i include 1 fail safe meal a week (if not more) but i refuse to stop cooking the things we all need because they wont eat it, if they don't eat it they don't get anything else.

5. Involve them - i ask DD1 what she wants for tea, she often helps make it. We have chickens now and since getting her own eggs she will actually happily eat an egg or use it to cook things.

These things don't always work, DD1 still eats less at dinner time than most kids i know, she's a giant PITA and at times wont even sit at the table, so if she doesn't come to the table or refuses to eat she goes to her room or must leave so as not to distract DD2.
It's a work in progress but we are getting there, Taco's for dinner tonight too