Listen and imitate ... how? there are lots of ways available, people here have said great alternatives! ^^

Listen to American accent conversations or songs Watch American TV shows or movies ( with subtitle ) Speak to an American ( native speaker ) Listen to words pronunciation in American accent ( http://www.thefreedictionary.com/hello ) Good Luck ^_^

Speak through your nose, rather than with your mouth.

Find a language partner, get a program that allows you to speak with them verbally, and practice.

watch more american movies without subtitles, so u can better concentrate on pronunciation. singing songs also helpes. 加油！