Remember to always try to avoid

pronouncing an English sound by mimicking it with a close one in Chinese.

It sounds like that your problem is with the two vowels
'ei' (as in train, rain, rake and ray) and long 'u' (as in spoon, moon, mood, etc). These two are among the hardest for us. It took me about half a year to truely know how they should sound, and I'm still not sure if I can always say them correctly.

Check these websites out:

http://evaeaston.com/pr/uw-U-pattern.html
http://www.uiowa.edu/~acadtech/phonetics/#

As much as I respect and appreciate the replier's hospitality and efforts, I disagree with some of the earlier replies to this post, which could be misleading.