If the girl is not into it, the guy will know. And if the guy knows the girl is not into it he thinks she is not enjoying it. That means he is not enjoying it either. A combination of hand stroking and mouth sucking on the tip works best. Do some humming to show that you are enjoying it. Swallow, spit, or let it spill out if you want, by that time the guy really doesnt care just as long as it keeps going LoL.