Everyone can speak English clearly
The first thing you should know is that everyone can speak English with a clear pronunciation, everybody has a mouth, a tongue and lips. Everyone can speak every language because there is no difference between human; we are all born with the same mechanism.
English environment
If you really want to improve your English pronunciation, you should be surrounded by English everyday, which means the best thing is to be in a place where we talk only in English, like the USA or Britain or Australia...
If you can't have this situation, you can do it by listening to English everyday, you can listen to English radio, or TV, you can also have some CDs in English...
listen and don't speak
This is one of the biggest differences between children and adults, you should find the solution to this problem to not only improve your English pronunciation but also your global English learning which means English grammar, English speaking...
When someone talks to you or to someone else in English you should not speak at the same time, you should listen, focus on his speaking, his English intonation, which words he stress, which words he doesn't stress...
Never speak when listening to English, always focus on listening first, speaking will come later!
Learn English subconsciously
You must know that you will learn English if you try, you will speak English like a native speaker if you don't stop, but the problem is always the time.
Never try to learn English in a short time, listen always to English and be sure that you will speak it one day, you don't know exactly when but you will learn it. Always listen to English and don't try anything else, you will learn English subconsciously, believe me!
Focus on listening
If you want to learn English or to improve you English pronunciation without listening to English pronunciation, stop right now!! You can improve your pronunciation without knowing what is really pronunciation, and you can't know what is English pronunciation if you don't listen to English.
speak slowly
Never try to speak fast, always try to speak English slowly at the first time, you will get faster subconsciously without doing anything, you should pronounce each word correctly, that's the goal!
Practice English
Always practice your English, read out loud, speak English with your friends, practice English everyday, this will help you to know the correct way to stress syllables.
Speak out loud
Never be quiet, always speak out loud, this helps you a lot if you are shy when you speak English.
Record yourself speaking English
And compare your speaking with a native speaker, you will see the mistakes you've done, you will improve your English pronunciation.
Have some English friends
You should have some friends with the same goal as you, friends trying to learn English, you should practice your English with them, tell them if you find something useful to improve your English or if you find a new method, they will also tell you, they help you when you get bored, it's really one of the best things to improve your English.