Given the huge and growing number of advertisements for penis enhancement products on the Internet and elsewhere I'm surprised I haven't received more questions like yours. The array of products and methods available is truly mind-boggling - everything from herbs, lotions, pills, pumps to exercises and surgery.
For the record, I know of no herbs capable of increasing the size of the penis. Some herbs such as ashwagandha (withania somnifera), from the Ayurvedic tradition in India, are used to increase sexual potency in men. Asian ginseng (panax ginseng) has a long history as a sexual energizer for men but has no effect on penis size.
Another method widely promoted as both "natural" and "ancient" is called "jelqing" and involves stroking the penis. Proponents say you can gain up to four inches in length with this technique, but urologists attribute any change in size to swelling caused by bleeding under the skin triggered by the prescribed jelqing "exercise." Urologists also warn that in time, this technique could cause formation of scar tissue in the penis, which could interfere with the ability to get an erection.
Surgery is available to lengthen or widen the penis, but according to the American Urological Society there's no proof that either operation is safe or that they work as intended. The lengthening procedure requires cutting a ligament. As a result, about an inch of the penis that is held inside the body drops down so that it looks longer from the outside. The most length you can gain is an inch. After surgery a man has to wear weights on his penis to keep scar tissue from forming and can't have sex for at least a month. The widening procedure involves grafting of fat into the shaft of the penis.
The only other method I'm aware of that might help is the use of a penis pump normally recommended for treatment of erectile dysfunction. The pump creates a vacuum around the penis allowing it to engorge with blood so that it increases in size; this also helps with maintaining erections. Use of the pump over time may help one's penis get fatter according to urologists, but increase isn't permanent - it typically disappears when you stop using the pump.
The short answer to your question: save your money. Penis enlargement methods, natural or not, are definitely not worthwhile. Men considering them would be better off dealing with the self-esteem issues underlying the mistaken belief that size is what matters most.
Andrew Weil, M.D.