There is a quick way to find out if you have the potential to come while giving a blowjob. Suck the head of his penis and pay attention to how your throat is responding. If it seems to quiver or vibrate faintly in time with your sucking, you can do this.
As you are sucking, breathe deeply and rhythmically. Imagine that your breath is a circle of fire inhaled into your nostrils, filling your mouth and throat and exhaled through your genitals. Flex your PC muscle in time with your breathing and sucking. (Do you practice Kegel exercises? A strong PC is required.) Make intense eye contact with him as he nears ejaculation.
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You can also make it happen if you learn The Orgasm Loop, my no-fail, even no-hands orgasm technique.
It's a thrilling way to come—especially if your orgasm and his happen simultaneously.