Bill,
I have been fortunate enough to have caught 2 koi on the fly. They are a real spooky fish. For this reason I enlisted the help of a friend who bait fishes for them quite often. (He has a pond containing over 60 koi that he has caught from the "wild". Probably from people who got tired of them and released them into the stream.
Flouracarbon leader is a must for fishing for these guys. Seaguar brand is the best in my estimation. 2 pound test is what I use. It is actually pretty tough. It is expensive, but well worth the money for the results.
My fly of choice is a bread fly made up of white glo bug yarn. If you go to the carp anglers group website (www.carpanglersgroup.com) and look up an article written by Lowell King (kingfisher) titled "Creating flies for Carp" it explains how to make this version of the bread fly. I tie them on a #4 mustad 3366 hook. If you PM me your address I would be more than glad to send you a few.
Stealth is also very important when fishing for these spooky fish. Stick with it and I'm sure you will be successful too. Good luck there sir!

Ray