Spend more time on oral. Start by eating her out and don't stop until she's screaming and shoving your head away in ecstasy. You'll make her so sensitive down there that its literally a sensory overload for you to continue. It's fantastic. Make sure you focus on her clit though, that's where they are most sensitive to oral and as much as you desire to shove your tongue down into her pink and soak up all those juices, you have to keep that to a somewhat minimum.
After she's had hers, let her suck on you for a while and when she's got you close to cumming, change pace again. Go back down on her again, she'll come even harder the second time. After that, you can have her suck on you again or you can get down to business and even if you do bust your nut early, she still was able to come several times. It's a win win.