well, as im about to finish my career as pharmacyst and biochemyst. i can tell u that there are some methods.
the ones based on jelks and masajes and vacuum work. but everything is slow. all helps to enlarge vascualr tissue, but nerves grow slower than that. 1mm a week maximum in those periferic nerves. so its easyer for the body to grow in girth than in length.
the HGH inyections are great for that. but as also gives more girth than length...
what i meant is, if u want to get 3 to 5 inches permanent... it will take more than 3 years to achieve that by exercises... and u must be regular... its like the gym dude!!
but at the end u wont loose it!