Clear Speaking

I've been wondered why I couldn't understand some people's English even their pronunciation don't have strong accents. So, I also have been comparing the differences between the two; the people whose English is very clear, and the people whose English don't clear.

Then I found some important factors for the clear speaking. That would be; the pronunciation, the volume of the voice, the intervals between the words, and the sentence structure. For native English speakers, the main cause of confusing English (mainly to international students) would be the last two; the intervals between the words and the sentence structure.

It's difficult to understand if one speaks long long sentences with almost no intervals. Most radio personalities use long and short phases or sentences evenly, and put intervals very effectively, so even they speak very fast, I can understand their words.

...May be I also should train my speaking too, because I often fail to express my feeling to the others.