One common problem women performing fellatio make is using only their mouth to repeatedly stroke his penis. They continue doing this until either it works, or they get a sore jaw and neck. A good "blow job" should not be too repetitive, should include stroking using a hand and the exploration of his testicles, thighs and (if you and he like) the anal region. By mixing up your style, you allow your muscles to relax, you can avoid getting a sore neck and you can improve your performance.
How you mix it up should depend on what your partner's likes. Although there is a lot to say about fellatio that can get the job done in a matter of minutes, it is the longer ones that usually result in more intense orgasms, and coincidently are remembered. (Click here to read about stroking techniques).
It is a good idea to keep an eye on his scrotum (his testicle "sack"), as it is usually a good indicator of how close he is to coming. As a guy gets closer to climax, the skin on his scrotum tightens and pulls his testicles towards his body to warm them up. You can let this happen on its own, or help out by stimulating his testicles with a hand, tongue, or mouth.