Lots of people will tell you to watch American television shows to hear authentic examples of American English. That’s an excellent idea but you can take it a step further to get some serious pronunciation practice.
Instead of simply watching your show, choose a character to imitate. As soon as your character starts to speak, try to say exactly what they are saying. Speak right along with the character!
Mirror the character’s spoken English for about 15 seconds at a time. This will really help you get a feel for the way American English speakers use syllable stress, sentence stress, linking and intonation patterns.