Hi
Does your reflex comes during local anesthesia? If not, that could be a solution: numbing the sensitive spots in your mouth.
Here are some other tips:
1) If you need to make a short action that might irritate you reflex, for example take impressions or X-ray photo, you can rinse your mouth with Listerine or something else with a strong taste (like salt water) so that the strong taste will stay in the mouth for couple of minutes and cover other sensations in mouth.
2) What can really help is not moving any muscle in the mouth especially not the tongue and not the palate muscles. If an object is just touching the muscle instead of moving aginst it, than the irritation is much lessened.
3) As Gordon stated, practicing at home is also helpful, you can also do it with a spoon instead of with a toothbrush.
4) Laughing gas has also a beneficial effect, but not in every case. It is worth trying though.
5) trustful relationship with the dentist. That's the most important thing. If the dentist knows you and where are the sensitive spots in your mouth, and feel trust and ease during the treatment, than the gag reflex improves tremendously.
all the best