I intentionally changed my accent to American when i was a kid. The R's were the real problem and it took years to master. I had to learn how to translate from one accent to the other on the fly. And that's why I sometimes pronounce things strangely. Most often it involves R's because i change “uh” to “RR”. For example sometimes I pronounce indicator “indercator”. Heh. Another thing to watch for, but it's not as obvious, are pronouncing TU sounds. If your accent is like my real accent you probably pronounce tuesday with a little CH to it. Closer to tchuesday. See also stupid becoming closer to stchupid. Americans don't do that, so make sure you just pronounce the TU part without any extra bits. But that is generally pretty rare and shouldn't come up too much. Anyway that was a very long way to say the R is the bitch of the bunch and when it comes down to it you'll just have to practice. It's a very very subtle thing, too much or too little and its very noticeable for a native speaker. Just remember that too much R is as bad as, if not worse than, too little.