1. NO TEETH. Scraping the penis is bad.
2. What they all said - get the whole shaft going (as much as you can comfortably take)
3. keep the suction going.
4. just listen to him - see if he's responding to your efforts, or not.
5. keeping a hand on the base of the dick will help him keep hard.
6. to avoid gagging, try to relax the throat as much as possible, take the dick down your throat slowly & gently.
7. to quote Sly Stallone, "the balls, cradle the balls..."