When you practice with a dildo, pick one of moderate size that has an as-smooth-as-possible surface and that doesn't have an overly big knob at the end. Don't use dildos made from PVC or TPE, the material is porous and thus unsanitary, use silicone dildos instead (do clean those). And then I guess, try to relax, take it slow (really, really slow - for the beginning, try to put it in and just leave it there for a couple of seconds, instead of going straight to fucking your face) and don't overdo it. Try to practice often for a short time, rather than rarely but for a long time.

I found that it's easier (less gag-inducing) if I put the dildo on a surface and move my head onto it from the top, rather than moving the dildo with my hand while being upright or lying down. Luckily, that's a more realistic scenario for oral sex anyway!

These worked for me, hope at least one of them work for you, too!