For those of you who aren't aware, it is perfectly legal to target wild carp in Japan. A friend of mine caught one the other day that was pushing 10 kilos or 20 pounds. This was on a fly rod. It took about 40 minutes to land. I had one about the same size break me off. As for techniques, with a fly rod use a black wooley bugger, any bonefish pattern, or a small clouser. The fly pretty much has to be right in front of their face before they'll hit though. Watch for them to suck the fly in, set the hook, and hold on. They hit like a large mouth and run like a steelhead. I recomend at least an 8 weight and a reel with a strong drag. They can be a little tippet shy, so flurocarbon is probably a better choice than monofiliment. 5 X is about the lowest I'd go though. Also, depending on where you are fishing snakeheads, trout, and other assorted fish are sometimes fun extras.
Good luck and have fun.