Recommendation Letter for My Teacher

I am now writing a recommendation letter for my former adviser at Haverford College. It sounds a little strange, doesn't it? That's because my current adviser and another professor are nominating him for the best teacher award at ASA. And I am asked to write a letter of support.

I actually find it difficult to write a letter for my former teacher. I have a lot of good things to say about him, but I don't want to sound too expressive, exaggerating, or even worse, phony. The best way to go about this is probably to pick up some specific episodes about my interaction with him and explain why they are indicators of excellent mentoring.