Our rules are: Stan has to taste everything on his plate, and he's not allowed to say anything is 'yucky' (because someone spent time making the food for him). By 'taste', that doesn't mean he has to chew and swallow. If he's reluctant, we go through these steps together.
Look at the food - what colour is it
Sniff the food
Touch the food - is it hot/cold/hard/soft etc
Pick up a bit of the food and kiss it
Stick out his tongue and lick it
Put a piece of the food in his mouth and take it straight back out again.
Put a piece of the food in his mouth and eat it.
He has to get to the second last step (put it in his mouth and take it out again). If he's really reluctant, he doesn't have to put it in his mouth and eat it (but usually he does). We have done these steps about 100 times with broccoli, and he has just started to eat broccoli. Score. (Sigh that it took so long but hey).
We also have a song that we do from Yo Gabba Gabba, and if DH or I say we don't like something, we go through the taste steps too and try it. DH discovered he doesn't HATE yoghurt this way. This was actually great for Stan to see.