Get comfortable! If your not in a comfortable position you will find it difficult to relax your jaw.

The knack to avoiding to much taste when he cums is to keep him toward the back of your throat and don't pull back when he does(even though that's the instinctive thing to do),the less that gets over your tongue and around your mouth the less likely you are to taste something you don't like.
Also avoid breathing through your nose for several seconds after.
Breathing through your nose is about 40-60% of taste. Try it with other things you will see what I mean.

there is also flavoured condoms, and definitely alternate between hand and mouth to save your jaw.

Don't let this seem intimidating, there is always the option to spit, although do try and do that subtly into a hand towel or something.

But as previous posters have said, just have fun with it.