Practicing chord changes

In order to practice chord changes effectively, you should start slow. And the most important thing is that choose a strumming pattern which is very very simple and you are very much used to that strumming pattern. The motto behind it is that while practicing chord changes, you want your whole focus on the chord changes and not on the strumming pattern. So chose a strumming pattern which you can play without thinking about
it. So now your whole focus will be on chord changes. Try to follow the concept of common fingers and strictly apply them to your playing.
 It is not always necessary to strum the guitar while practicing chord changes. You can just try the changes on your guitar without strumming the guitar too. And this kind of thing can be practiced when people around you don't want to get disturbed by your practice.
Remember, be slow and accurate and better results will follow.