I think the biggest problem is that, sometimes you cant find a similar pronunciation in English, or there are some slight differences between the pronunciation you think and the true pronunciation. I meet the same problem when learning English.
1. you should strengthen your Chinese soundmarks, the pronunciation is combined by those soundmarks. Especially pay attention to, "sh",'ch','c',"s","en","eng"....It is not easy to introduce the differences with characters.
2. Watching CCTV news is a nice way of learning the pronunciation. The comperes are the tops among China and they wont add dialects in their Chinese.
3. Accent is a problem too, I have heard many foreigners talking Chinese, the accents are strange, so you should concentrate on the tone of the sentences. They are quite different from English.
Wish these will help you.