Sit in an easy-chair and lean back a little.
Grab your penis behind the glans (head) between your thumb and bent index finger. Stretch it as far as possible in the direction of your erection.
Now squeeze your PC muscles (pulling your penis toward your body; see Kegels) while stretching with your hand (note: you can also tilt your pelvis to increase the pull).
Hold at maximum tension for a moment.
Next, slowly relax PC muscles, maintaining the pull from your hand, until you have reached the fully stretched position again.
Repeat this exercise 20 times.
Now do another set of repetitions, increasing resistance so that you can only do 15 repetitions.
Perform every set to the point of total exhaustion of the muscles. Increased resistance is the key, rather than increased number of repetitions. After completing the sets, hold your penis in the starting position and stretch it as far as possible, two times, holding it stretched for a minute each time. After that do a short massage until your penis is soft and relaxed again.
Regimen: day on/day off. Do these exercises one day; do nothing the next day. You will be able to increase force over time as you get accustomed to doing the exercise.
you can experiment with variations of this - pulling in different directions, making circles as you pull, etc.