Giving a blow job ...

So my husband loves when I blow him..... I notice I can only do it really good when I am really in the mood.. when I'm not in the mood i gag so much cus his cock is huge and i feel bad cus we never get to finish the way he wants to because i cant handle it all... i wanna be able to give him the best blow job and not have to be really turned on..any tips on helping with my gag reflexes?