Use a lot of tongue, and focus on the head with your mouth, don't shove the cock into your mouth, and use your hand a lot for his shaft.
USE YOUR HEAD to move your tongue rather than your tongue muscle. Not that you aren't to use your tongue, but if possible, delegate the work to your neck muscles rather than tongue. Use the tongue for the tiny, specific movements, use your neck for the larger movements. This'll keep you going for longer, as well as more efficiently. Brace-specific advice I cannot give, I'm hoping the first bit covers it.