there's this book called Tickle His Pickle. You should check it out. Tons of great pointers in there.
start by running your tongue slowly up and down his penis (on the outside) then use the tip of your tongue to tease the glans (the small area behind the head) this is electric.. when he's fully erect suck the head then move slowly down, then thrust faster and faster whilst keeping maximum contact. we like your mouth to be hot and wet like a vagina so expect some thrusting.. also cup his balls in your hand..
Do it slow! Try to take it a bit further in your mouth every time you blow on him and go slow to control the gagging reflex. Another thing that might help you is blowing on him while you're lying on your back and your head is off the edge of the bed. This position will allow him to go further without gagging you (well, not as much as if you're kneeling or else) and it will be more comfortable for you. Also tell him to relax, trying to force his way down your throat isn't going to help at all (in fact, it can physically hurt you as much as it can repulse you). I hope this will help.
Practice (as in daily practice, at least twice a day is best) will really help, My other half has a huge dick and even tho i could always take cock deep into my mouth and throat i still had to train myself to be able to take his, its now possible for me to take it all, not constantly because he literally ends up fucking my throat so he has to come out so i can breath :') you need to learn to control your gag reflex, practice sucking his cock and then as you take it deeper do a swallowing motion. this helps with taking it deep. if you still cant do it then lie on the bed, hang your head over the edge and let him fuck your mouth, warn him that you may gag on his cock and if you do that he is to take it out right away! (trust is essential) but doing it regularly will make your gag reflex less intense and eventually it will be fine If you need any other tips them PM me, im quite a pro at giving head to big cocks if i do say so myself x