Putting some lube on your lips or on him can allow your mouth to go up and down smoothly which helps the whole process.
Keep in mind, this training must take place over time in order to be effective.
Fake it by wrapping your hands around his girth, in front of your lips, so your hand feels like an extended mouth.
Lifting your legs can alter your spine's alignment so that your throat and mouth are in a straight line.
This creates a pillow-like cushion for him to thrust into and protects your gag zone.
It is meant to prevent things from going down your throat that aren't supposed to be there, and to stop you from choking.