I feel your pain, literally. My jaw kinda sucks; I've had more than a few days when it hurts to eat because of a yawn. Obviously this means that giving blowjobs can be tricky for me, but I've learned a thing or two and I hope some of these tips are at least a little helpful. Keep in mind that I'm no sexpert, I just try to be patient and open-minded. Every man is different, and someone out there may think I'm awful at blowjobs. I have yet to meet him, but he must not be a very fun guy.
/cracks fingers
Alright. Get comfy with a tall glass of water, because my best most crucial super good advice is to get everything nice and slobbered up before even putting a dick in your mouth. Yes, that means try to be hydrated and be able to produce a lot of saliva. Or just use lube, however I have yet to find anything that doesn't taste like liquid plastic. I say this because the more lubed up he is, the easier it is to use your hand(s) pleasurably along with your mouth and to be able to relax your mouth over his dick while sucking lightly with as little friction as possible. Less friction = happier penis, and less work for you. Harder sucking = more strain on your jaw.
Seriously though, waterboard his junk with a seemingly unreasonable amount of lubrication, and don't leave those balls hanging. They want attention too. A hair or two (if he doesn't manscape) isn't going to kill you, and it's usually cleaner than you think down there. Remember all that spit? Yeah, your hand(s) still feels pretty good. I usually get the best reaction from sucking the balls gently while swirling my tongue, stroking with one hand in a somewhat rotating fashion to keep things going up above (think accelerating and decelerating a motorcycle in traffic?), and using a couple fingers from my other hand to massage the perineum. (thanks, Google) Or just grab his butt because butts are awesome and also because that whole production may feel like a "patting your head and rubbing your stomach" kind of thing.
When you do take him all in, try once in a while to get him as far into your mouth as you can and use your throat to kinda swallow, or at least flex some muscles back there. It sounds weird, but I do know that this doesn't hurt my jaw and it feels quite nice for my partner. I usually let my jaw go slack, stick my tongue out a little to protect him from my lower teeth, and curl my upper lip just a bit to cover my upper teeth. No need to deep throat, though; that's still a bit too difficult for me, so using the muscles in your throat is the next best thing, I think.
You can do a bang-up job a of blowjob for your man if you take your time. And I emphasize taking your time because, not for every man but for most, having fun, staying relaxed, enjoying what you're doing, and not putting so much pressure on yourself to make him happy makes him orgasm all the harder, better, faster, stronger etc. It's infinitely better for your jaw if you don't stress and tense up!
That got quite a bit longer than I thought it would. If anyone has any thoughts, comments or corrections, I would love to hear them. I have a communicative SO but sometimes he's just too nice to say anything constructive.