Stop stroking and squeeze your balls tight. Slap them and squeeze them.
Are you sure you really want to cum? I'm not convinced. Beg me more!
Say, "Please let me cum Dr Jergins!".
Squeeze those balls harder and say it again.
I'm very pleased with your progress; continue stroking, this time you are only allowed to use the tips of three fingers to stimulate yourself though. You can go as fast as you want...relax your pelvis and feel your cum bubbling up.