Here is a link to a site I have been a member of for 10+ years. The links go to two exercises that I modified to make work more effectively. What I did was tweak the regular and V-jelqs. I started off around 5 1/2" of mid erect girth and am at 6 1/2+" now. There is no spam from this site, no advertising emails, etc. Just helpful information from good people.
Not proven , but increasing testosterone levels does help alot! Lift weights, eat healthy, good sleep, a good fuck and you are bound to see a positive change!