From Volley cam

http://hdtenniscamera.com/tennis-video-training

high lights of the article are:  it's a step by step way to improve your strokes

Step 1:  Identify a technically “perfect” stroke on YouTube.

We’ll call this the “control” video.  It should be of a pro hitting in slow motion with a good view of the player from head to foot.  This video of Thomas Berdych’s 2 hand backhand is perfect:

go to the site for this video

Step 2:  Film 3-5 of your sessions on a ball machine.

Set up a camera on the baseline.  It doesn’t need to be my VolleyCam, or anything expensive, but high definition will be a big help.  Hit a few hundred balls at a time and return home to compare your stroke to your “control” video (in my case, Berdych hitting backhands).  Identify one thing to work on during your next session (ie, shoulder turn, extension of left hand into court, etc.).  Keep it simple, and focused:  you can’t learn it all in one session.  With each session, compare videos and “check off” each element (shoulders, knee bend, footwork) until you have a fundamentally solid and repeatable stroke.

You have to hit about 10,000 tennis balls to learn something new.  With video analysis, I’ve cut that number in half.

Step 3:   Real world testing.  More video.

Find a partner or instructor good enough (and patient enough) to isolate your new stroke.  You’re about to find out the vast difference between “ball machine” confidence and “real world” confidence.  The ball will be coming at a varied pace, angle and height.  You will struggle.  It will be frustrating.  At this point, some players give up and return to their flawed but familiar strokes.  But if you’re diligent about filming, comparing to the test video, and working on just one thing at a time, you’re reducing a large problem into a series of smaller, more manageable problems.  And you have the benefit of video to verify your progress.