Practise on your man/Master when you are aroused. Really wanting it, sexually, might help you forget about the reflex and make you relaxe the most. And as mentioned above: repetition.... I`m sure he will allow you to practise your skills on him
I have a question in addition; can you hurt your throat if the cock goes too far down too many times?
While the feeling of deep throating is amazing, the clogged up, eye watering and burning throat experience that commes with it make me worry a tiny bit about the parts down my throat.