Hello Roy,
Is not always possible to extract milt for some males, at least whithout risking to damage them.
You can anesthesize the fish in an aerated bolw adding drops of oil of cloves. Then, wrap the male with a damp towel and gently squeeze the vent to extract milt.
One thing you can do to stimulate the process of sperm formation is to inject the fish with Ovaprim (from Syndel). I have used this synthetic hormone and it works very vell with males, they produce copious milt.
You can buy it on line. Follow the link below and search "spawning products"
http://www.syndel.com/home.html
One word of caution, one of the "males" I injected was a Sanke that I bought from a japanese breeder the previous year. He assured me that the fish was a female, but was hard to believe. Growing on, the koi retained a clear male body shape, and I thought the breeder was wrong.
Well, when I anesthesized the koi (assuming he was a male) and tried to extract milt what I got was a bunch of still inmature eggs!
Good luck,
Diego