In an effort to sound more American, some of my students try to imitate the speech of their native speaking friends, colleagues or spouses. While this is generally a good idea, you need to be aware of the fact that some native speakers may not be such great role models for you. Be careful of those who mumble (have lazy lips and don’t fully enunciate each sound).
Generally talk radio show hosts and news broadcasters are good role models to imitate. Action movie heroes are usually not!
Also, pay attention to the differences between casual and formal speech. The speaking style that your friends use in casual situations may not be appropriate in a professional environment.
For example, I had a student who used the word “hey” instead of “hi,” even on her voicemail at work. “Hey, you have reached Jane” just doesn’t sound appropriate.” It’s OK to use “hey” as a greeting for your friends but not for business communication.