yep, i remember my first b.j... giving one, i mean!
well, i had the same prob. the thing is, you shouldn't open your mouth all the way and keep your jaw locked. your moth should be relaxed so this thing won't happen. if your jaw or neck does get tired though, give it a rest and try just licking/switch to a hand job. when you're rested up, go back to the blowjob.
like goonarms said, with time, and practice, that shit will be obsolete!