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American English Advice
For American English, differences between British and American English can be related to pronunciation, vocabulary and a bit of grammar. However, if you have a British accent, so long as it is quite neutral, this might be relatively easy for many/most people in America who also have a standard accent at least, to improve.
To try and improve your American English listening skills, you could try listening to news reports on American channels such as CNN on the internet or also going to the Voice of America learning English site: "http://learningenglish.voanews.com".