Every month, I create a 'Passatta' of just about every vegetable going. Spinach, toms, Peppers, brocolli, carrots, celery, etc, etc. I then blend it, and put it through a muslin cloth and have a lovely smooth sauce which I add to all of their food. We did this form the start really, not because they were picky, but we thought it a win - win situation even when they did start eating veg. So now, for example, we'll give them grilled fish, with rice, and Brocolli, peppers and carrots and then pour over a very good portion of the sauce. They've got to really enjoy it.
I know it is also probably against the grain, as I have read books outlining it's not good to do, but we have always said to the kids, that what we put on the table, they eat. That they will stay at the table until it is their bath time if they don't eat it, thus eating into their activity time. We've also added that if there is something new to them, they have to try it, even if it is just once.
They do kick up an awful fuss every now and again (Mango was really tough to get them to eat, as if it is slightly too ripe, it is slimy), but they do operate to the rules and seem to be much better now than they were at any stage.
We looked at this from the angle, that we'd rather they understood we were going to be rigid on this, that healthy eating is a must do, than worry about forcing them to miss the fun things by making them stay at the table. Sometimes they make a stand, and we have to respect that, but in the main, it has worked.