Also...
1) Lie on your back on a table, like a dining room or kitchen table.
2) Hang your head over the edge and open up.

This totally straightens your throat and he will (probably) slide right in without touching the nerves that invoke the gag reflex.

Eventually you'll learn how to position your head so that your throat straightens from between his knees, kneeling over him, lying beneath him... however he likes to take you. Occassionally he will bang against the nerve plexus as you make adjustments...

...and eventually the nerve plexus will allow harder and harder touches to occur without reflexing.

Alternately, I found it helpful to lie on my side on a bed, and have my girl on her knees on the floor. I make her lean over the bed and "reach" for my cock. This stretches her neck out, also straightening it... (Oddly enough, for us, this works better if she is to my left than to my right... probably due to slight variations in cock and throat angles.)

You can add variations by tying her down to the table... or locking her wrists behind her as she leans across the bed... Nice for both the dom and sub mindsets.