The biggest tip I can give you is - don't ever think about it ever again. Your head is your worst enemy here. If you get into bed with your girl thinking "gee, I hope my dick doesn't x" - your dick will x twice.
Second tip - cut porn and masturbation for like a week as well. You'll have no problems getting hard in any position, then your mind will finally shut up about non doggy positions.
Thank you sir. Although, no masturbation for a week seems terribly difficult as I'm away from the girlfriend for 10 days. MY LUCK, but probably worth it like you said (did a while back GAF had a no masturbation contest?)