The other day I was with a class of adult students who worry about their English pronunciation. We were doing an exercise to practise linking sounds. For example:

"My sister's son is seven years old."

I was trying to demonstrate that some of the 's' in the sentence are pronounced /z/ (as in "years old") and that when there are two 's' ("sister's son") we only pronounce one of them.

But then it struck me that a lot of correct pronunciation comes naturally. If you allow your mouth to "make the decisions", your pronunciation is correct. For example, it's almost impossible to pronounce the 's' in "years old" as an 's' without pausing the sentence – the mouth naturally makes a /z/ sound to make the sentence flow.

In some cases, I think that trying to remember pronunciation rules can be counter-productive. Instead, trust your mouth to find the right sounds!