Watch 15-20 minutes of local/world news. Listening to news for a short period of time will acclimate the ear to standard English pronunciation, which is what journalists are well-versed in. This is really the best way to learn new words, idioms, and of course, the standard way of speaking in American English.
Watch American movies by breaking down scenes. Many non-native speakers learn American English by watching American movies and old television shows. Of course, you have to watch with a purpose. Select a least three films spanning over three decades (1970, 1980, 1990) and one from the current decade. You should spend at least two weeks on each movie. Break the movie down in 15-minute intervals. First, watch without captions. Try to understand the context of the scenes. During the second viewing, watch with English captions. This time try to follow the dialogue. The third viewing should be much easier to understand and follow. Write down any new words or phrases you hear. Try this with all the movies. Your vocabulary and listening skills should improve over the coming weeks and months using this method.
Learn how Americans stress words when they speak. English is a time-stressed language. Many non-native speakers often report confusion when they hear Americans speak. In most cases, it seems as if Americans "swallow" their words. Actually, when a native American speaks, they are unconsciously stressing specific words over non-important ones. When practicing American English, try stressing content words (i.e., adjectives, verbs, nouns, adverbs) while gliding over the less-important "function words" (i.e., prepositions, articles and auxiliary verbs). Words like "in the" are spoken together as one word, like "intha," or "to the" becomes "totha" in American English. The most important thing to do is focus on listening to words you can hear when observing native Americans speak.
Use a voice recorder to tape/record American speakers. Use a digital voice recorder to record C-SPAN interviews from TV or the internet. Typically, people who appear on C-SPAN use standard English. Also, there is a wide cross section of guests from government, intellectual institutions and artists from around the country who share different regional dialects. Try to record at least thirty minutes. Write down key words and phrases, and tape yourself mimicking the speaker.