don't make orgasming the goal of her performing oral on you. just enjoy it. it's pleasurable whether or not you cum. but if you're having trouble have her incorporate a handjob into it. one thing my girlfriend does is suck and lick for a bit, getting me all hot and bothered and then she jerks me off with the saliva as lube till it dries up and then she starts her oral regimen again. By the fourth or fifth time if i'm in the right mindset i can cum from her doing this. i don't think i could cum at all just from oral stimulation, but combining oral and manual stimulation works wonders.

but like i said. don't go into it expecting to cum. go into it wanting to enjoy the feel of the experience and the emotional feeling of the closeness that it brings. Only a couple of my girlfriends have been willing to give oral at all so feel lucky you have someone that cares for you enough to do that, and let her know that's how you feel about it.

Less than a minute ago I answered this in another thread.

In addition, learn where his hotspots are around the Corona rim, include them in your caresses as well as the Frenulum. In addition, learn how he strokes himself and learn to mimic his movements. You can use them when giving a hand job as well as in combination with oral. Finally, encourage him to give feedback either verbally or non-verbally on how he is reacting and then take your cues from this.

I'll add that everyone is different. I, too, rarely reach orgasm strickly from oral sex. It's a matter of conditioning (safe-sex dictates that you don't ejaculate in someone's mouth) and technique (the other person really has to learn what pressure/speed works for you). VP and Doc both have good suggestions.