Another good thing to do is to get an old cassette recorder or record yourself using your computer, then compare your sounds to those on your instructional material or check out some of the calling on sites like http://www.callingducks.com
or the sound files on the RNT website.
As far as learning to blow your call to finish ducks, most instructional videos only teach you how to operate the call they don't really cover actual hunting scenarios. The Duck Commander's "The art of Commanding ducks" while not the best video for learning to blow a duck call does have some useful pointers towards the end on working ducks. Some duck hunting videos (non instructional) have some good sequences also.
Another thing that is worth doing is to get out your duck call in the off season, head out to an area frequented by mallards and practice working ducks into shooting range. I often take mine out when fishing and I'll hunker down and call at passing mallards for a bit of entertainment.