To circulate your sexual energy, simply tighten the muscles around the genitals and perineum as you inhale. 
Visualize, or feel, your sexual energy flowing up your spine to the crown of your head. (The image may help you visualize this.)
Now, as you exhale, visualize the energy flowing down the front of your body to your navel. Store it there. This step is very important according to Mantak Chia, author of Taoist Secrets of Love: Cultivating Male Sexual Energy. Otherwise, the energy can build up in your head and cause headaches.
Repeat.
You may initially feel more aroused. Take as many breaths as necessary to reduce your level of arousal to a comfortable level.
For an expanded version of this exercise, see these excerpts from a talk on "Transmuting Sexual Energy" by Swami Satyananda Saraswati.