To simulate the feeling of lips sliding over the penis head: Lube up your erection and hold the shaft in one hand. Put your dominant hand's index and middle fingers together horizontally, palm toward your face, in front of the penis head. Push the head into the gap between the fingers. The fingers should be relaxed, not tense. Now pull back and do this over and over the top third of the penis. Feel how the fingers conform to the penis's contour as they slide over the head and back. As you do this, position your other hand behind the "lips," and grip and squeeze the head as you push the penis in.