The best way to improve your American accent is to be surrounded as much as possible with American English, and to speak with native speakers. This isn't possible for those of you who don't live in North America, and even people who do don't always have a lot of opportunities to interact or the confidence to try out their American accent when speaking with Americans. In that case, using YouTube, tv and movies to supplement the pronunciation and intonation practice you're doing will help.
Using Media to Improve Pronunciation
First, turn on close-captioning or subtitles in English for whatever you are watching. Research shows that pronunciation actually improves when people follow the written words as they are listening. This is partly because you can see all of the words, which can help you notice the details of how they are being pronounced.
Next, choose a listening target. If you've been working on your z sound on the ends of words, listen to the media with that in mind. Can you hear the z's? This will help you be more aware of when you should use the pattern in your own speech. Or perhaps you are working on the TH sound. Listen for the words with TH and focus on watching the speaker's face. Can you see the tongue placement between the teeth? Or, select a vowel sound you are working on, and try to notice every time you hear a word with that vowel. You can have a list of words with the vowel in front of you as you watch to help you with this.
Lastly, you can slow down the playback speed of YouTube videos. I recommend listening at 75% playback speed, because it's not so slow that it sounds really unnatural, but it's slow enough to hear pronunciation details that you may miss when listening at full speed. (See my video below for instructions on how to slow down a YouTube video.)
Using media to improve intonation
Use my Real Talk Videos (playlist below) to help you listen for intonation patterns as well as linking and reductions in fluent speech.