I got my sun coral from Jim at Coral Paradise. This is what he suggested and I found it effective.
I feed at the same time daily. Whatever I was feeding the tank, mysis, brine shrimp, Red's, Rod's I used a kids medicine dropper and slowly, put some food in front of it so that it would blow over the coral. I did this 3 or 4 times at around 10 minute intervals. Then I would feed the tank like normal. After a week or so it started opening up by itself, around feeding time. Ever since, I don't have to feed them special. Now, they just open at feeding time and eat with everyone else.
Worked like a charm for me.