A metal knitting needle is great to scratch with, there are different types of knitting needles, so make sure you get one that is smooth, metal and cannot break the skin. Another thing is using a hair dryer on a cool setting and blowing it down the cast. This will cool the skin, dry it out and reduce the histamine, which is what causes itching in the skin. Of course, the last thing is to take benedryl (diphenhydramine). Benedryl seems to work the best, but any antihistamine will do. Remember that antihistamines will make you tired, so be careful if you need to drive. Knocking gently on the cast helps a little too!
There are also a few things you should not do! Don't try to use any powder or topical creams! Don't use anything that is not completely smooth to get down the cast. Don't try to bend or twist the cast to get to the itch.
Remember that most of the time the best way to get rid of an itch is just to ignore it or to maybe give it a short scratch or tap. Continuing to scratch and scratch just increases the inflammation and the histamine it produces.